Oneplan sends debt collectors after me after I cancelled policy over year ago.

This is one of the worst companies ever. In February 2017 I cancelled my policy, because as we all know, this is medical insurance, not medical aid, so it is useless. On 4 February 2017 I sent them an email to cancel my membership as soon as possible. I also called to confirm that request was cancelled. They advised they received my request. The last debit order went off on 1 March 2017. ALL payments were received. a month after that I got sms stating that I did not pay them. I phoned in and asked what is going on, because I cancelled my policy. They said they made mistake, will correct. Phone call was recorded where I cancelled policy and the one where they admitted their mistake. Today 5 May 2018 I get a SMS from debt collectors, I owe Oneplan R200. Ridiculous. I phone Oneplan, give my ID nr. Then they say I did not cancel policy, they cancelled it due to non-payment. LOL. They have a recording where I cancel the policy. The day when I cancelled, there was NO mention of any outstanding amounts. It is ******* to raise invoices after membership has been cancelled. It is even worse to hand over this debt. Something drastic must be done within this company. They are really losing the plot. Urgently void this invoice of R200 which was *******y raised, and remove me from your debt collector's list.