A President can be impeached. The problem is BO/BS is not President because he has never proven he is a natural born citizen per the requirement in Article 2 Section 1 of the Constitution. The appropriate charge is treason. Being found guilty of that charge would negate any documents he has signed including all bills passed by Congress during his illegal occupation of the Oval Office and Executive Orders.
For now, he is as de facto president as Rahm is mayor of Chicago, even if both the law and I agree with you. His natural born citizenship, any and all fraud around it, can be made part of the charges of impeachment, because regardless of his right to be there, he occupies the office. And he can be impeached.