[Marco van Herwaarden]

Asuming that they are still in the awaiting email confirmation usergroup, and haven't done anything (posting for example) the following query SHOULD do the trick. Mind that this is not tested. Make sure you backup your database first.

[sql]DELETE FROM user AS u, useractivation as ua, userfield as UF, usertextfield as utf
WHERE u.usergroupid = 3
and u.userid=ua.userid
and u.userid=uf.userid
and u.userid=utf.userid;
[/sql]
If you are using a tableprefix, add that before each tablename.

After this is finished, you should optimize these tables:
[sql]OPTIMIZE TABLE user, useractivation, userfield, usertextfield[/sql]