If your cellphone runs apps, look in your app market for Mr. Number (if you live in the US). It will block unwanted calls and SMS. I have it, it's free and works great. If you don't live in the US, look for an SMS blocker, and if your phone doesn't run apps, contact your service provider.
I would also advise you to check your phone bill-sometimes scammers use SMS messages to steal money from other people's accounts.

These people aren't calling me but they are texting me like crazy