Allowing debit order to go through

I canceled a debit order with Absa bank from Mtn as I am currently disputing a R30000 cellphone account, on the 28/09/2015 but they still allowed the R30000 debit order to go through even though my balance of funds wasn't nearly enough to cover that amount. How does anyone allow such an amount to be deducted from my any institution allow this after the debit order was canceled and there is not sufficient funds available?