sharp tu-tv252h keeps powering on?

We have a second hand TU-TV252H bought from a local charity shop. Managed to get it going with a download manual but when we turn it off some 10 minutes later it comes back on. Tried turning it off on the handset and on the box itself. Rang Sharp who said they had never heard of this problem before and that was there solution to the problem. Fat lot of use that was. Can anybody please help. I see somebody cannot turn theirs own!!!!!!!!!!!