Put them under armed guard who are instructed to do what’s necessary to protect the painting.
Let these protestors stand there for days, if necessary. But to be humane, provide them with plenty of food and water so they don’t get hungry or thirsty.
Then wait for nature to take its inevitable course.
That’s called “performance art”.
I disagree.
They should remove the painting from the frame and preserve it.
Then they should relocate them and the frame glued to their hands to a windowless empty closet the size of a large phone booth and a locked door.
Fill the closet with pepper spray.
Toss in a huge hornet’s nest filled with agitated hornets, and perhaps for good measure, throw in a naked Alec Baldwin and an angry, rabid badger.
I’ll bet their hands would break free in no time at all.