Withdraw any party support from them, kick them from the party, and fund their opposition. The Democratic party can absolutely do this when they need to try and stamp out anyone to the left of the blue dogs.
West Virginia is extremely conservative and risking a seat can be quite dangerous when you are trying to govern against literal fascists
Before Shelley Morito got elected in 2015, the last time West Virginia had a republican senator was during the eisenhower administration. And if Manchin votes as a republican anyway why does it matter? Like he’s straight up come out and said he won’t vote for a Joe Biden pick for the supreme court.