How to Stop Phone Spam

Recently I've been getting these really annoying telemarketer phone calls on my cell phone (which should be illegal, in my opinion). A friend of mine said that every time he gets a spam call, he adds a contact to his phone called "Telemarketer" or something like that. Then, when a repeat number calls, you can see the name on the phone's screen and not answer. I suspect it might take a while to build up a library of numbers before this primitive blacklist actually detects a spam call, but the more calls you're getting, the more I think you'd find it useful.