Delete certain posts from a user with specific content?

[SmEdD]

DELETE FROM $table WHERE $userid, $column LIKE '%$string%';

$table = What table (post, privatemessages)
$userid = User id (first user is 1)
$column = What part of the table is the string in (message)
$string = Phrase or word

I'm not 100% that will work, that was off top my head. Also after you have to update the counts via admin panel.