[Andreas]

Easiest way
postbit_display_complete

PHP Code:

if (!is_member_of($this->registry->userinfo, x, y,z))
{
unset($this->post['signature']);
} 


Replace x, y and z with the Usergroups that should not be able to view Signatures.

Most efficient way:
- Use an SQL-Query to turn Sig display off for all Users in Usergroups you don't want to be able to view sigs
- Create a Plugin for Hook userdata_presave that checks if the user is in an unsergroup that does not have permission and then unset the view signatures option