MTN HUGE SPAMMERS

I have been getting spam calls from MTN on a daily rate of at least 10 calls per day carrying on right through the day to late in the evening requesting me to upgrade. I upgraded my contract in August last year for a 2 year contract, and the more I tell them that I have already upgraded, the more they will phone me. This is now getting highly irritating. Don't they have entries on their system that shows which upgrades have been done. Getting my information, it should show that the upgrades have been done!!