supanet are a member of ISPA, the Internet Services Providers' Association.
They are a bit like OFCOM, and describe themselves as "the UK's Trade Association for providers of Internet services".
They also have a code of practice that its members have to follow. If you have finished the length of contract you signed up to, I would guess that you can leave supanet with one months notice, and no further charges (not sure what their terms and conditions are).
But you can complain to ISPA, or at least get advice off them. See their website:
I am currently complaining to them about Orange broadband. If you do change broadband provider, then dont go with Orange, they are incompetent in the extreme...!