There is nothing unconstitutional or even immoral about "banning" Muslim immigration. (Which isn't what Trump proposes, anyway.)

1) Foreigners do not have Constitutional rights.

2) Muslims believe in the supremacy of Sharia Law, which is a system of law DICTATED BY GOD. It is whole, and complete, and comprehensive, and no man-made law has any validity. Thus, Muslims believe in the overthrow of the U.S. Constitution, and thus are ineligible for immigration.