Poor all-round service

I contacted Optivest a while back to take out Gap Cover Insurance. The consultant could not do enough to get me to sign. He emailed and called numerous times. It was only after I had signed all the paperwork that someone brought their poor track record to my attention. I decided that since I had just started paying, I would rather cut ties with them immediately. This has been an impossible mission as they just do not respond to any calls, messages or emails. Can I just go to my bank and stop the debit order. Very poor service for someone 'linked' to Old Mutual.