(2) leave large, annoying, or obscene messages; and (3) leave files which, when run, erase disks.
I believe this BBS is crash-proof. If not, I'll hear about it soon and fix it.
As to obscene, etc., messages, the best thing to do is delete and ignore them. Replying only encourages the vandal. If you ignore him/her/it, they'll eventually get bored and go away.
Trojan horses are impossible to filter out 100%. You can try running them with your main disk turned off. If you don't filter, do warn your users.