You're right; there is very little recourse except to try to set a good example. Response among opposing fans almost inevitably leads to escalation, which doesn't help anything and in fact makes matters worse. Sometimes the best way to teach young people that certain behavior is inappropriate is to NOT mirror it, no matter how wrong it is or how right you think you are. If you get drawn into a sideline confrontation, at some point you become what you're protesting against.
Unlike high school, in a lot of club matches there is no true "home" side; both teams are often on the road from varying distances, so assigning responsibility to the home team's staff doesn't always work; I would say, though, that it is the responsibility of each team's coach, staff, club, etc. to set the tone and expectations for behavior on the field and on the sidelines, and address parents and fans as a group or as individuals, as necessary, if inappropriate behavior is present.
If there's an issue, rather than trying to settle it on the sideline, report it to the coach--after the game has ended and the dust settled. I can say from experience that I don't actually hear 90% of what is said on the parents' side of the field. When you're focused on the game, everything else gets tuned out into background noise, and if the coach isn't aware, he/she can't work to fix it. If there's no satisfaction there, then report issues to club management. Again, they can only work to correct what they're aware of.
Character and image, in my opinion, are more important in the long run than wins and losses...if I had to choose between winning a trophy and having an official after a match say, "I always enjoy officiating your team's matches," I'll happily take that second one.
