Not a lot you can do to stop it ... but posting your email address on a forum is an open invitation for more of the same. Email Harvesting Robots are a great nuisance.
Once your email address is on someones list the only way to eliminate the spam it is to delete that email address and use a new one ... be careful who you give your email out to and never openly post it.
I had one email address that was getting spammed @ 100-200 a day ... how it was harvested was easy, some little **** posted it on his forum, I simply deleted that email address.
I have several "throw-away" email addresses and Google.mail is great for that purpose.
Report all "spam" to your ISP