He can't.
Let's get with the program, folks.
The dinner is sponsored by the White House Correspondents' Association (WHCA) and is organized and financed by the association.
The White House has NOTHING TO DO WITH THE EVENT'S STRUCTURE ITSELF except for some peripheral matters such as consultation on the president's speech, special guests, security, etc.
Leni
Trump can’t call it off but I can’t think of one reason why any Republican should attend. Can you?
He doesn’t have to attend. Let them have their fun if they want, but ignore it.