Depends on whether or not the collector fights the case law that says that unlisted debts in a no-asset CH 7 are considered discharged. Chances are that they're not going to put forth the effort or money to pursue it unless you owe them a LOT of money.
So, send them a cease and desist letter, cite the case law you need in the letter, and tell them never to contact you about it again. If they persist, you CAN have your case re-opened. You might also want to see if your case is officially closed yet. If its not, you may be able to add this creditor a bit more easily.