Snowy, I will bet next month's rent you have only made a verbal cease-comm, which cannot be proven and never sticks with debt collectors. Send one on paper, USPS Certified, and they have to listen, or else!
If you are receiving repeated, misdirected, and/or abusive calls from a third party debt collector, you may find relief in the provisions of the FDCPA, the Federal law which regulates collector behavior. Learn your rights at FTC-dot-gov. See also your state laws for additional support.