how to disconnect Nokia N97 from an unwanted network?

I have a Nokia N97 phone and I am having difficulties with disconnecting a network that i have not subscribed for (AT&T). I have no clue how my phone connected to it what so ever because I have my own wifi under another company. I got charged $150 jus for internet; which I still dont have a clue why I am being charged especially if I barely use my phone. Other than calling AT&T and talking to them is there another way that i can disconnect it?
 
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