I left BT at the begining of Febuary yet they still charged me for the entire month of which I only got £6 back, so I cancelled my direct debit only to then recieve another bill for March. That one they admitted was a mistake so I didn't have to pay, but if I hadn't cancelled the DD they would have still taken the money.
Not to mention the emails they kept sending telling me I was near my broadband usage limit and would I like to upgrade, when I hadn't even used a quater of the allowance!
I'm glad I left!