Using hierarchical keywords is a safer practice in Lightroom, because the keywords and hierarchy for each file are kept primarily in a central database or "catalog" and can be instantly modified there for your entire photo collection. But Bridge has no such database -- nothing at all but a simple list of keywords to show in the Keyword panel. The keywords and hierarchy for each photo can only be written directly into the individual photo file or its associated XMP file. And if you modify your hierarchy in the Keyword panel, or discard it entirely, the original scheme is still written into the files you've applied it to.
What's more, whenever Bridge reindexes that file for a search, it may use that old hierarchical scheme to replace keywords it concludes are missing -- even if you've since changed or deleted those keywords. In Adobe's eyes, that's a feature, not a bug. They're helping you out. Thanks, Adobe!
To put a stop to this paternalism, the first thing you must do is TURN OFF hierarchical keyword functions. In the Keywords panel of Bridge Preferences, turn off ALL options: "Automatically Apply Parent Keywords," "Write Hierarchical Keywords," and "Read Hierarchical Keywords." You can still create a keyword hierarchy in the Keyword panel and apply parent keywords manually, you just can't let Bridge apply them automatically.
If you are working with Lightroom as well, do NOT use it for keywording. If you do, depending on your settings, you'll either have inconsistent keywording between Lightroom and Bridge, or you'll have Lightroom writing its hierarchical keywords to your photo files, where Bridge will read them.
These steps will stop Adobe from making your problem even worse, but you still need to clean up the mess already made. To do that, you have to understand where those pesky keywords are stored.
With Adobe, that's in three separate keyword fields in a photo's XMP -- the collection of data fields inserted into the photo file itself or into an associated XMP file. (In Bridge or Lightroom, the XMP file is normally hidden, but it's plainly visible in the Mac's Finder.) The fields are officially called:
dc:subject -- This is part of the IPTC Core metadata. It's your primary keyword field, an industry standard, and any modern keywording program will read and write it. You want all your keywords and ONLY your keywords to appear in this field.
iptc:keywords -- This is the keyword field found in the older, "legacy" IPTC data set. You don't really need it except for compatibility with older apps. For safety, Bridge synchronizes this field with dc:subject.
lr:hierarchicalsubject -- This is where Bridge stores info on hierarchical keywords -- and where Lightroom does too, if it writes that data to individual files. So, this is where Bridge may find "missing" keywords it then decides need restoring.
The Metadata panel of Bridge displays and lets you directly edit the keyword fields of both IPTC Core and legacy IPTC -- assuming that viewing of these fields is turned on in the Metadata panel of Bridge Preferences. Bridge makes sure that changes in one field are reflected in the other. You can also add or remove keywords from both these fields together in the Keyword panel.
This third field is proprietary to Adobe -- so while Adobe provides no help with it, neither does any other app. But there is one tool that can help: Phil Harvey's ExifTool (www.exiftool.org). It's a Perl library that's called from Terminal's command line, and you can use it to remove all data from this field.
After installing ExifTool, you would type or paste the following command into Terminal, followed by the path of the file or folder you want to process, and then press Return or Enter.
exiftool -overwrite_original -xmp-lr:hierarchicalsubject= -r
For instance, the command with the path to my photos folder looks like this:
exiftool -overwrite_original -xmp-lr:hierarchicalsubject= -r /Volumes/Docs/Works/Photos/
If you're not sure how to write your desired path, you can just type or paste the first part and then drag the folder or file into Terminal.
Note that, if you have a lot of photos, ExifTool's processing takes a LONG time. At the end, Terminal will show a summary of results, but your only feedback meanwhile will be when ExifTool issues an error message or warning. Aside from that, the only way to make sure ExifTool is running is in Activity Monitor.
Also note that ExifTool will SKIP a file if it finds any error in the metadata. (I had this problem with close to 300 files that had started life as Nikon NEFs, been edited in Capture NX 2, and later been converted to DNG -- a sequence that seems to consistently produce the ExifTool error message "Bad MakerNotes offset for NEFBitDepth.") You can tell ExifTool to process the file anyway by adding "-m" to the command -- but then you risk even worse corruption.
You should now be able to search your photos and remove the keywords you don't want. But notice I said "should." In my own case, I STILL had unwanted keywords reappearing.
The problem, as I came to understand it, is that the Bridge Metadata and Keyword panels, because of corrupted metadata or whatever reason, sometimes fail to write your edits to the file -- AND BRIDGE DOESN'T NOTIFY YOU. The panels show the changes, and they're probably in Bridge's cache, but they're not in the photo XMP. The next time you reindex the photo -- say, on running a keyword search the very next time you open Bridge -- all the old keyword data returns to Bridge's panels.
Luckily, I discovered a solution to this. With any problem file, IGNORE Bridge's Metadata and Keyword panels. Instead, make your changes in the File Info dialog, accessed from the File menu or the contextual menu. This command not only writes your data reliably to both of the visible keyword fields in the photo's XMP, but it also fixes whatever problem was present, so that you then CAN safely use the Metadata and Keyword panels for your changes.
Actually, I need to qualify that. This fix works if you're using a LATER version of Bridge. Applying this method in Bridge 2018 fixed only SOME of my problem files. The same method in Bridge 2020 fixed ALL of them.
At least, I THINK they're fixed. That's how it looks so far. ;)