Best way to fire a band member who isn't working out? -Dirty Work.
Simple answer: Privately and Face-to-Face.
...of course there is (always) more to say, but the above answer should be part of everyone's basic human code of dignity. I know you kids all like to do everything by text these days but, trust me on this one.
There will be times when a private sit-down isn't possible or safe. Maybe there are drugs or, worse yet, lawyers involved? In that case do whatever the highest possible high road might be. Do it 'by-the-book' and be done with it.
Don't get me wrong "face-to-face" doesn't have to mean 'one-on-one' ... Stack the odds in your favor, Meet 'in private' but make sure the rest of the band is there too, and if a show of strength is required, maybe have a few roadies in the room too. Lay out your reasons for kicking him out; be specific, try to make it sound like helpful criticism and not like a bone-stabbing. Give him/her the chance to respond, but don't get into an argument. Make it clear that the topic is not debatable. Help load out their equipment and try to shake hands at parting.
No, I don't expect it to go that way, but it is something to shoot for.