What are they debiting for? Is it an auto-payment for something?

Right off the bat I would send a registered letter instructing them to stop and contest the charges and keep a copy of the letter w/registration document proving it was sent. The date they recieve the letter will be an important point in time. It puts them on notice and starts the clock for future actions.

I would not wait for a reply but call them and try to resolve it- if attempts fail the registered letter is already in motion- in the event of a legal dispute that letter will prove to be a valuable document. Hopefully it will not come to that.