Altech keeps disappointing and taking money!

Upon expiry of my 24 month contract at the end of April 2015, I gave notice of cancellation on 9 April after which I received a service desk confirmation that it was processed (ref631844). I furthermore confirmed cancellation at the branch in Grove Mall. On the 28th of April they proceeded to delete my number off the system instead of reverting it to prepaid as requested, after which a Vodacom assistant called the call centre who confirm that my number was deleted after notice of cancellation on 9 April to be effected on 28 April. Altech then proceeded to debit my account with double my monthly installment at the end of April, even though a price increase would only take effect if I renewed or continued my existing contract with them at the end of April as my existing agreement and terms thereof was still valid. As per the agreement, the monthly installment will remain as is for 24 months. I then submitted several queries, but is yet to receive any feedback/comment. I received a further invoice for May 2015 for (again) double my monthly installment even though cancellation was effected on 28 April. When will they stop ripping people off. Worst service provider ever!!!