I feel like your idea would hurt NG more than help it, since most of the new sign-ups are spur of the moment decisions anyway. New users who are attracted by the BBS are likely people who want to reply to a topic or want to make a new thread of their own, so they create an account on the spot.
If there was a waiting period, or some long process to go through to sign up, I feel like most people would just say, "screw it" and abandon the prospect of posting on the BBS, likely abandoning Newgrounds as a whole as well.
As long as the mod is descriptive in the 'Reason for Ban' portion of the new user's ban, the user should get the hint pretty quickly.