Home internet return?

I have requested multiple times for this to be cancelled and removed from my bill. The people at 611 told me it would not be terminated until I take the modem to a store to turn it in. I have been to 3 of my local stores and they refuse to accept the modem. Telling me I need to mail it back in the return box they send me. They refuse to send me a box to ship it back. Please help me out of this contract before I have to cancel my t-mobile all together.