Harassment at its best!!!

Constant harassment from this debt collection company, call everyday up to 5 times to setup a debit order, and every time I said to the agent I don't want a debit order. I have a payment arrangement and paying monthly via my credit card, so please STOP with the calls. Further to this, one agent that called, just forgot her name, insisted that they will keep on calling until I set up a debit order. So from when can someone force you to set up a debit order.