A while ago somebody at invisioncommunity.com asked how to delete all his tags after the conversion from vBulletin.
Unfortunately there's no built in way to accomplish this in IPS4, but it's still quite easy to achieve by truncating few database tables.
So, if you want to get rid of all your tags and prefixes, you have to truncate
- core_tags
- core_tags_cache
- core_tags_perms
- core_search_index_tags