How many distinct IPs are involved? Could be a lonely script kiddy or a pissed off member.
If only a couple IPs, an update to your iptables would keep them out. If a lot of different IPs are involved, then I'd be more worried as it could be someone with a bot net at their disposal, or they are at least advanced enough to spoof the IP. You'd never be able to keep up with it without some scripting of your own.