My forum from time to time will jump to as many as 200 guests (usually no more than 30-70).
We have found they come from domains that have no link to us at all.
We think that these "random guests" are infected with spyware or something that is making a pop-up show up on their screen, showing my forum.
They visit some random link, as far as one year old.
How can I stop these particular guests from coming to my site?
Screenshot below: