Or sobbing uncontrollably. Or bothering the stage management.
Other than kick their ass? It depends on the situation really. If you’re in a professional theater where this person is getting paid, fire their sorry ass. There are loads more people who would be willing to take their place.
If this is for a school production, understand that it may be from a lack or respect. Take the actor aside and talk to them one on one. Ask what’s the matter, then tell them that they need to shape up or they’re out of the cast. If they leave, fine. Bump up the rest of the cast you have- sometimes talent can be overridden by someone who knows how to follow directions. Also, if you bump someone up, you bump up a bunch of other people as well, until you can find the smallest part possible, then find someone for that role. It’s easier than simply flat out replacing the person. It may get a different feeling to the piece, but better a mediocre performance than one that is absolute crap because of one person.
If anyone else has any advice, feel free to throw it in!