Harassed for service that I didn't receive!

I app**** for wireless internet on April at webafrica and debit order went through at the end of the month, during May I contacted them asking when are they going to install but they told me that they are still waiting for the payment to reflect on their system, second debit order went through as well but nothing happened, then I decided to reverse my money now they are harassing me with the calls that owe them but they never installed the internet, how can I pay for the service that I didn't get! This people are ******** and harassing people!