Your son probably won’t respond well to your blocking her number. With Facebook, Twitter and other communication options, he will most likely find another way to communicate with her.
If you want to solve this problem with discipline, you can institute cell phone-free hours in the home. You put all of the phone chargers on one counter, and make a house rule that all phones must stay there between the hours of 11 p.m. and 7 a.m. so that everyone can rest without distractions.
Infatuation is a powerful force. Your son may have to work through some of this on his own. Let him know that you are there for him and that you support him and will comfort him when he is hurt. Explain to him that she is not treating him in a loving, respectful way. He deserves to be appreciated in a committed fashion.
Keep an eye on his disposition so that his natural sadness and pain don’t worsen into depression. If you see any signs of teen depression, encourage him to talk to a counselor. Some teenagers need the impartial voice of a counselor to help them out of emotionally damaging situations.