He has total authority over national security and if they doesn't sell you, he has this to back him up:
8 U.S. Code § 1182, entitled “Inadmissible aliens,” reads:
Suspension of entry or imposition of restrictions by President
Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation, and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate. Whenever the Attorney General finds that a commercial airline has failed to comply with regulations of the Attorney General relating to requirements of airlines for the detection of fraudulent documents used by passengers traveling to the United States (including the training of personnel in such detection), the Attorney General may suspend the entry of some or all aliens transported to the United States by such airline
Trump has already used 8 U.S. Code § 1182 to limit refugees from Syria. The federal courts put a stop to it. Yes the SCOTUS later gave him a very narrow victory on it in the end. They also made clear they would review any use of it in the future. If he used it to refuse asylum seekers at the southern boarder they would rule against Trumps in a 5/4 ruling. Trump is doing everything he can do to secure the border and people like you and culter are sitting at the sidelines bitching. I got news for you, the only people you are helping are people like pelosi and shumer.
The problem is, if one person were to be shot. The Republicans would probably impeach him.