I agree you should avoid freeing up spammers email addresses, but although running a SQL query for 'level' and 'last login' isn't a problem, in the OP's case it may return several thousand rows of non-sequential user IDs.
I'm struggling to work out how you would get those ID numbers into the users group link table to associate them with a 'banned' group?. I also can't help thinking it would be better to delete those records to improve efficiency of the database?
A 'User Purge' (with a banned/delete option) would be a good core feature IMHO.