Basically, money talks, and the problem with amateur events is that the umpire doesn't have the power to do much more than verbally warn parents and crazy fans. If I'm working a higher-level game, like a high school or collegiate event, I might have the authority to eject a fan, but even that requires actually filing paperwork on the incident after the game, and that simply isn't worth the hassle. I'd much rather wait it out and later drink away the memory of your crazy, purple, screaming face.
One shot for every psycho dad who tells you to shoot yourself.
On the other hand, the only options left if a fan won't calm the hell down are to hope for intervention from somebody -- anybody at all -- or to call the police. I've never had to do it, but that's what the local umpire union advises. Some places like Florida have at least somewhat streamlined the incident reporting process, but after you submit the form, it's entirely out of your hands. Not to mention how you may have to deal with that team again later on, and as mad as that angry fan got at you before, they're going to be worse the next time.
So how bad can it get? Well ...