Your best bet is to take it to another repair center. Tell them that you did not do anything to the phone, insist that the port never acted right. If that does not work, take it to the manager. After that take it to the bbb and rma it to htc. Do not ask for the repair center, just say it will not charge and sprint will not fix it. Get an rma number and send it in.
HTC Mobile Phones - Customer Service - Repair Information
Call them and tell them they have to honor the warranty. If they tell you to go to sprint, tell that that you want to do a warranty repair. If they insist on you returning it to sprint, get a contact number of the person, and have sprint talk to them.
It is replaceable under warranty. Just be nice.
OP, why are you always complaining about htc repair? You did not create two accounts to repost what has been deleted have you? Because that would be against forums rules.