If they're really unused—that is, unattached to any items—they'll be purged by the client automatically. They're not purged if you just use the website.
How often will the tags be purged? My tags are 'clean' in the library itself (the unused tags have been deleted together with the unused entry's), but in the webversion it's still quite a mess.
Thanks, Dan. Nevertheless, I've been waiting some days, and have restarted the browser accordingly more often. Nothing changed.
It seems the caching is aggressive indeed, as unused tags from a test-import (deleted afterwards) are still there in the online-view of tags and also, by the way, when adding tags to entries.
It would be great if there was a way to get rid of these unused ones, because they are pretty useless in my case and getting on my nerves while working.
The tags are stored with your session, so if you restart your browser but restore the previous session it will not clear your tags. If you don't want to lose your session you can force an uncached reload in most browsers with ctrl-shift-R or cmd-shift-R.
They still show up when adding new tags in the client, you mean? That means they haven't been purged locally. If you delete a tag from the tag selector (by right-clicking and choosing Delete) and sync, any unused tags should be purged. If the tags are still linked to items in the trash, they won't be purged.
Upgrade Storage
It seems the caching is aggressive indeed, as unused tags from a test-import (deleted afterwards) are still there in the online-view of tags and also, by the way, when adding tags to entries.
It would be great if there was a way to get rid of these unused ones, because they are pretty useless in my case and getting on my nerves while working.