User rights

I have a problem with a user rights? They have power user rights an they shouldnt have. The rights I set for them was just a user only the other day there account got lock for some reason and thats when I notice it. I am the only admin for the local computer so no one should have been able to change the rights. I can ad an remove rights to the account but when I try to remove the power user rights it gives me an error (the specified account name is not a member of the local group) I have been every where in the computer that i can think of please help