Fixed! Go to the fix.
I had same problem a few years back; didn't want voicemail because we have a nuisance-blocking answering machine. At that time I was told to phone Pnet tech support as it was not possible online. A long wait as usual but very helpful when I did get through.
Re: Unable to disable Voicemail
Thanks for getting in touch.
I'm sorry to see you're unable to disable your voicemail service, we'd be happy to carry this out from our side if you want.
Feel free to send me a PM with your full name, confirming you are the account holder, full address and your mobile number.