Unbelievably poor service and administration by a large Company Netstar.

Sold my vehicle 4 months ago, contacted Netstar to sell the tracker service to the new owner which they did. Formally terminated my contract with them, however debit order still taken off the following month. Have now had 4 interventions with them and each time they have struggled to explain why it is happening. They confirm transfer to the new owner. They say I shouldn't be being charged. Apologies profusely, then the next money deduct the money again. I am at a loss with what to do.