Paying for a service I will not be using after August

I tried to cancel my services online the last day of July for it to come to an end of August as i will be moving to a new address which has no adsl in the area. I was unable to do this online as the cancellation tab did not work at all when clicking on the "cancel my services" it opened a new tab with the message of "Session expired" after I logged in many times after that, the message kept coming up. When i called a few days later seeing that it still did not work i was told that I am still liable to pay for the full month of September this is totally unacceptable