I definitely have this same issue. Except in my case it is MUCH worse. I have 238,642 tags that have been generated from the hashtags in hundreds of RSS feeds; most of which I never read.
I have also seen extremely slowness in file save and open dialog boxes - I believe caused by this functionality interfering with Finder itself.
Now I need to first find a way in DEVONthink to clean up the RSS feeds that I have so that I can eliminate the ones I never use or read, then clean up the tags. I have already told it to stop creating tags from keywords or hashtags, but that only keeps things from continuing to go downhill. It does not correct the current problem.
If there is anything that could be added to the product that would prevent this problem, I would like to hear it. Perhaps storing tags in such a way that they’re in DEVONthink only and not part of the Finder or the OS? Or at the very least a way to have the conversion of hashtags and keywords into tags be prevented with RSS feeds?
Any crazy thoughts? 