Well first question is what version you are using and a url to your forum.
Also If you did in fact create a new group with special permissions for new registrations you need to goto:
Control Panel » Registration Settings
And set the default group for new users.
Also doing so with a new user group you can limit there access/ privileges after x number of posts. After that they are promoted to the next level group that allows more user permissions. If you so choose.
Or just not allow signatures at all for selected groups by setting it to 0
