I just bought an Iphone 4 i tried activating it but sprint says its already activated on someones account. how can i take it off their act
You need to either call your phone company or the apple company and tell them your issue.
make call to company..or search for web
If you bought it second hand you need to contact the original owner and they'll call their carrier and have the phone released.
The only way is to contact the person who sold you the phone. Before they sold you the phone they should have made sure it was deactivated.