NBC reportedly has no direct ownership of a broadcast facility. As such it has no broadcast license. Thus the president cannot revoke it.

If they did have a broadcast license, it would be an abuse of power for the president to cause its revocation directly or indirectly because he is not in the normal chain of review and decision of such a matter.

Further, if he did it without following ordinary procedure, including hearings prior to revocation, that would be a violation of due process.

Most importantly, his motives, which appear to be to censor political speech would be a prohibited basis for action even if revocation where otherwise possible. Behavior of the sort is the hallmark of a dictator. It is wrongdoing of a truly grand nature that cuts at the very heart of what it means to live in a free society.

What if his charge that they were making up the news was true? Well, the Sun once plead in that in open court as a defense to a civil lawsuit and the government did not come for them.

The law has not fully developed the furthest reaches of the balancing act of interests between the right to be free of slander and the right to speak without restraint. A case such as this might define that furthest reach. 

Or is slander really involved at all. Whether Kelly called Trump a moron is a claim about Kelly, not Trump. Further, it might entitle NBC to have Trump submit to a mental exam as part of discovery in litigation. A great many professionals have suggested Trump would not like the outcome of such an exam.