I see this all the time. The problem comes from those people in power that don't care. It starts at the top and when there is little to no support it makes it hard to keep and attract people. People can go to McDonalds and make more than they could umpiring, and why not? They don't get berated and yelled at for doing their job.
I have tried a lot of things in the past. Code of Conducts, better training, etc. What I have found that works really well is to make sure youth umpires look the part, attend coaches and umpire trainings, and that all parties sign an in depth code of conduct. A youth umpire in full uniform and who attends coaches meetings shows that they have a commitment compared to Joe Schmo in a tshirt and shorts. I make it known to my coaches that my umpires receive in depth training, have thorough evaluations, and take a rules test. In my rules meeting, I show the coaches what it is like to be in a training session and what our test questions are like. Out at the games, they see me work with young umpires during the first few weeks. There are swift punishments for coaches, parents, spectators, and players who violate our code of conduct. My facility supervisors are trained to spot problems before they get out of control and make sure that they do not get out of hand. I still have a fair share of people that don't conform to our rules, but those people are few and far between. Our issues have decreased dramatically since we have instituted these measures. It's not perfect, and it can become a very heavy workload, but it is a start and it has helped.