MTN ******s money from vulnerable customers

I've been a MTN contract holder for almost 5 years now and I'm disappointed at how they handle their contracts. I do not understand how they continue billing a customer for monthly services even after they have barred the customer's no. for being in arrears. And when you call them to cancel a contract that has run it's course, they'll say they can't cancel/terminate the contract because it's in arrears. Which is quite confusing...

1. My account is in arrears, I get that. It's my fault.

2. You continue billing me for supposedly monthly services offered. But what services should I be paying for when my no. has been barred and is no longer in use? I'm not receiving nor using any of those seevices yet I get billed for them as they continue to "expire".

3. I call to inform MTN that I am the contract had run its course and should be cancelled/stopped to prevent additional charges and I'm informed that it can't be done since my account is in arrears. So basically I must just accept the accumulation of bills of services I'm not receiving for a number that's no longer in use? I don't get it. Where is the logic behind that?

I can now afford to settle my bill but I need MTN to not bill me for their system not being able to recognise when a contract has run it's course and to know that I'm not liable for unfair, unnecessary and exploitory charges.

PS: I always paid MTN a little extra than I had to and the "little extra" was never subtracted from my next payment. Where did that money go?