My question is: How would or could a President Obama ban concealed carry
The president has no power under the constitution to ban concealed carry as a general matter. What he can do is encourage the congress to pass legislation and to sign the legislation.
There are narrow areas where the executive branch could act without specific legislation (and the president might be able to order them to adopt rules). For example, federal agencies could ban (and in some cases, have) the carrying of any guns in certain areas (federal buildings, national parks) under their jurisdiction. Such banning would be based on legislation passed by congress giving the agencies the authority to regulate conduct on the premises of property owned or controlled by the federal government.
But a general prohibition of concealed carry by fiat of the president? Nope.