[kh99]

I've seen people suggest that you simply remove the rows from the user database, and I think that might work (especially is the users have no posts), but it doesn't remove everything. What I usually suggest is something like this: create a new (temporary) usergroup, then execute a query like this:

UPDATE user SET usergroupid=X WHERE username LIKE 'imported_%'

where you replace the X with the id of your temp group. Then go to "Users > Move/Prune users" and delete all the users from that usergroup.

As always it's a good idea to have a db backup before doing things like this.