Unless you're on the run from the law, the police can be a good resource when being harassed by some creep.

I will second that google voice is a great tool, and add the you can easily change phone numbers with pay as you go cell-phone plans. Put a new sim in the phone, change the google voice forwarding to the new number, and you're good to go.

Good luck. Years ago one of my past girlfriends had a female stalker trying to ruin her life, and I know how aggravating the calls at all hours can be.