In slow-pitch, there are usually no "game administration", so the umpire may have to do the ejection, and ask whichever team the fan is associated with for help in having the ejected fan leave. I've actually sent fans to the parking lot in slow-pitch, and told them if they come back in during the night, I would call the cops and have them arrested for trespassing.

And Dave, before you say it, yes, we do get SOME fans at these games.