[Krofh]

You're getting there with the second one... the query is close, but I think the end should be:

if (strpos($post[signature], "Blah")&&strpos($post[signature],
"BlahBlah"))
{
$db->query_write("UPDATE user SET usergroup = 'bannedID#' WHERE
userid=".$post['userid']);
}

I think though that bbuserinfo gets the information of whoever's viewing the page, not the poster, so you probably don't want that in there. You don't need the "!== false" bits, it'll return true if it finds the string "Blah". The && is a logical AND, so the way this script runs is if it finds "Blah" AND "BlahBlah" in the signature then it'll ban them... if you want it to ban the user on either of them, you need a logical OR (||), so it'd be like this:

if (strpos($post[signature], "Blah")||strpos($post[signature],
"BlahBlah"))
{
$db->query_write("UPDATE user SET usergroup = 'bannedID#' WHERE
userid=".$post['userid']);
}

If you were to be more specific as to exactly what you wanted (usergroups and strings and everything), it'd be a bit easier for us to help you write code for what you're doing.