Apart from actually checking the guestbook every so often, there probably is not a solution to a product no longer available. I guess as with most websites, yours like ours suffers from spam and unwanted attention, but that is the price to pay for having a popluar website I guess

I dont tend to use the guestbook now but when I did, I used to check it regularly, then just delete the offending post and remark with something like:
"Deleted again you loser"
It may not have been politically correct but it seemed to have an impact somewhere