Quote From screamingaddabs:
Just the volume of spam. I try to highlight it when I see it with the abuse button, but there is often quite a lot of spam. I have no idea how you might sort this though...
Some boards have a delayed registration system so that you register but cannot post until your registration is approved. This might prevent some spammers as IP checks can be made to check if someone's a known spammer, or at least delay them posting till daylight hours when they can be more easily dealt with if they do start spamming.
On the subject of the 'abuse' button - it's a funny way to label it. When I hit it I'm half expecting it to auto generate an insult to the OP ;)