Sig Spam - How to stop it?

Hi,

We have been getting an increasing number of signature spammers on our forums, and they have been getting sneakier and sneakier, they seem to sign up, post in a high ranking thread, then come back later (sometimes weeks) and add a link to their sigs.

Does anyone know of a way to combat this? I remember you had some kind of signature add on a while back, but I'm not sure if this is what I need?

Any suggestions are welcome.

Thanks.

Please check What countries do you ban, totally