Spammer visitors

I've noticed that some of my traffic is spam, and it is from these links:

- xhttp://adsfg.info/hytrin/
- xhttp://jaegu.info/gris-peg/
- xhttp://jaegy.info/trileptal/
- xhttp://jaegu.info/zelnorm/
- xhttp://jaegu.info/penlac/
- xhttp://jaegu.info/chlorzoxazone/
- xhttp://jaeg.info/chlorzoxazone/
- xhttp://www.ilpharm.com/viagra-patch.php
- xhttp://jaegy.info/avita/
- xhttp://jaegy.info/videx/
- xhttp://fgrfg.info/ferrlecit/
- xhttp://jaeg.info/nexium/
- xhttp://adsfg.info/cleocin/
- xhttp://jaeg.info/arimidex/
- xhttp://jaegu.info/nexium/
- xhttp://jaegu.info/imodium/
- xhttp://jaeg.info/imodium/
- xhttp://adsfg.info/rocephin/
- xhttp://jaegy.info/robitussin/
- xhttp://where-to-buy-viagra.0catch.com

I think it's due to my forum, I have a lot of spammers there, every day I remove their posts and block them, now I set up that all new registrations require my confirmation, because it was impossible, I had so many spammers. And I think some of my traffic is from spammers.

How can I prevent my site from this traffic?

Thanks