i hesitated too, only because i thought the onliners box listed the current members of the site and that in some way i "wasnt allowed" to join.
if your going to stop them starting a thread, you should put up a message saying "sorry, but you must be a member to post threads outside the general mapping forum! please copy the message you made (written below), become a member and post again!
NOTE: please to not post off topic threads in the general mapping forum to avoid becoming a member!
then underneath would be the message they tried to post for them to copy.