Refusing to release my line

After struggling for months with my fibre line dropping or being x20 slower than what I pay for, I have decided to go over to another ISP. Knowing how useless Supersonic is I made various phone calls and also sent emails to notify them about this so that I can serve my month's notice. My notice month was served and my account is paid in full yet they aren't releasing my line. They have cut the service, but refuse to release the line. Their support centre is unable to give me a date and time of when my line will be released. I can't believe that even leaving your pathetic service you still manage to ruin my day. Supersonic is without any doubt the worst company I have ever had to deal with.