There are some good signing up ideas here. I like the "name a datsun model starting with B". Although this wont stop the more desperate spammers eg. that farrakah guy who advertised NOS- he clearly put a little more effort into his scam.
On the other side of the coin, when I sign up for sites I don't often see myself being on there often or in some cases ever returning, my sign up is usually just to ask a question or two. Basically what i'm saying is, if you make it too hard, people wont bother, and I don't want that to happen. I came back because this site is so useful and diverse, and to keep it that way we don't want to make it too exclusive.