Billed After Contract Was Cancelled And Equipment Returned

I cancelled my contract effective 30 April, and I dropped off my equipment to their offices 2 May. Got an email and sms to confirm equipment was returned, and I still get debited R999 for another month?! I didn't even that amount of money in my account, so now my account is in arrears. What a nightmare of a company. Tried contacting them, but their stupid whatsapp service is as poor as their accounting and I didn't get anywhere. This happens two months or so after a massive billing error where they wanted to debit me for 3 months at once, equalling over R2000! I'm so glad I've cancelled with this company. Worst ever!