Like b.a said, IPB 1.3 is know for its many exploits...and you will never be far from another immature person to come along and exploit it if you carry on using it.
I recommend using SMF (don't really know why Hitch says that it is the worst...) or like Hitch says if you can afford it go for IPB or vB (IPB is far better suited to smaller communities)
I know the amount of exploits for phpbb are blown way out of proportion, and they do release patches quickly however i can't really advise using something that has a complete lack of features compared to other free softwares (SMF, MyBB...) and still has more holes than others.
Matt