Highly unprofessional

Highly unprofessional people who call and harras me endlessly for an x-ray account which was paid beginning of 2017 and again towards the end. They refuse to update their system to show that the account has been paid (never mind double paid) and charged me interest equal to the account itself for interest and calls to myself (R25 per call + R25 for calling a cell nr). I get threatened by the callers and while I was abroad and could not take their calls (about 4 per day) they still added it to my account even though I sent them an email explaining that I was abroad and could not that calls, only email. They are still trying to get me to pay them the interest on the account even though it had been paid for part af the interest period they were charging me for. My bill was paid by my company when they retrenched me and it took me months to get a proof of payment from them after I stopped working there, only then could I see that they paid with an incorrect reference number. Is this even legal? Why do any clients pay them for their unprofessional services?