How to get rid of adware on a mac.?

I think I’m gonna go crazy from this audio popup that keeps on telling me that I won a 1000 dollar wal mart gift card. I need a Free Adware scanner and deleter for my mac. Can someone tell me how to get rid of it?

You should receive Malware removal support from a site like http://geekpolice.net/ if you have a virus or spyware.

I recommended it because they provide real fast support and a cure is guaranteed.

Mac Virus Advice