one thing could be you putting your e-mail and link out there for things in places where spammers can find you and more
Not a whole lot you can do but just delete or such.
I know a lot of people's guestbooks are JUST filled with spam because they don't check their guestbooks to clear it out even if have the image verification
I don't leave my e-mail and things when I sign guestbooks leads to spam there too.
