If he's interfering with your business then you have the right to ask him to leave. If he doesn't leave then you might have to get the law involved, have him barred from entering your establishment. Sounds drastic but it's something to keep in mind. Watch out for smashed windows and that kind of thing, however, as disgruntled employees can be very vindictive. Don't try to argue with him, just stay calm and tell him to go.
Luckily, I've only had one employee that I wanted to fire. She was lazy and kept getting "sick" when she was scheduled to work on Saturday or Monday mornings. Of course I'm the one who had to pick up the slack. It even got to the point where I told her that I didn't care if she was drunk, I would rather her call me the night before while she was out partying than wait until the next morning right before she was supposed to open. It was really annoying. I was close to my limit with her but she ended up leaving on her own, so that was good.
