Report to PTA. Don't email them though. Write a complaint, Cc it to all offices of PTA across Pakistan, then fax it to all of them. Fax numbers can be obtained from their website. It has worked for me three times.
PTA sends a letter to the service provider asking them to do the needful. The service provider checks the record and if your complaint is true, they'll call up that number and warn it not to do calls or send messages like that. They'll at this point call you as well on the contact number you provided, stating that the person has been warned. If that person does it again, write another fax referring to your old fax as well. PTA will direct the letter to service provider again, they'll verify and block that guy's number. Will call you again informing you about it.
At least that is how it happened with the case I forwarded . Good luck! All this took a week.