Terrible service, ripped off

I signed up with boomerang on a trial only to be told I could not get games unless my account was live. I went live on 8th December and still no games sent. I decided to add older titles and after more wait a game was finally sent (a game 2 years old) I sent the game back on 17th December and it is still today (29th decemver) showing on my rental list. I have emailed Boomerang twice with no reply. I want to cancel my service but feel they will say I have to pay for the disk what they have not received back so until I get a reply I am stuck.