Actually, I feel the opposite, I feel we shoudl pay them... Think of it, they download our pages for their DB, their users search their database and they send their users to our site, which tend to register and click advertising links which in turn make us money... For those of us who advertise anyway...
Now, if you want to stop them from visiting your site at all, you can, it's what robots.txt is for, just stop them from visiting your forums and never worry about them again (though you'll soon notice traffic decreases, and some sites depend on search engines for new users)
