Statements like this from Trump are jokes or sarcasm—he’s made similar remarks before, like suggesting Elon Musk be sent back to South Africa.
There’s no legal mechanism for a president to strip someone of their U.S. citizenship, especially a natural-born citizen like Rosie O’Donnell.
Trump’s gonna be Trump. There’s no changing that, however much we might like to. His supporters take him warts and all and his enemies are still his enemies (and hate him all-the-more for statements like this).
Trump knows that in a few days it will be forgotten and replaced with another tempest in a tea pot.
Sure about that? Through the Secretary of State, the State Department. I would be surprised if he could not. Expulsion.