Unable to cancel 12 month plan

I have now called the call centre on 3 occasions to cancel the 12 month plan on my Total Sports account. When I opened the account I told the in-store assistant I only want the 6 month interest free plan. She advised I call the call centre a few days later to request that the plan be changed. I did that and the agent advised that any balance would be converted to the 6 month interest free plan. with my first statement I noticed the interest and service fee but assumed the changes would only take effect on the next billing run. I was charged the following month again for the 12 month service fee + interest and called the call centre requesting that the transfer be done. This is now totally ridiculous. I called last month again asking that the 12 month plan be cancelled and I purposefully paid more than the required amount onto my account to account for the 12 month plan remaining balance. I want the 12 month plan completely removed from my account or I will close the account and go elsewhere!!!