There are some boundaries that can be set. Resistance to change is tolerable, but openly bullying newer members is malicious. If that happens again, make an example out of them. It will send a message to your other top contributors.
If you haven’t read it, “The No Asshole Rule” is a good book. The tl;dr is that a person’s value isn’t exclusively their own contributions. We also need to recognize that assholes make life miserable for others and decrease other group members’ contributions. It sounds like your most active top contributors are aggressive. Is that because you have no constructive top contributors, or because the aggressive contributors have made the environment too uncomfortable for reasonable contributors to participate? Especially if you haven’t been engaging with the community until recently, they may have felt like it was a lost cause and given up on trying to improve the community. And what of the new members they attack? Will that drive your would-be customers to competitors instead?
I’m speaking from personal experience. We dealt with a community member in our community that was actively organizing others to be hostile to the staff. Attempts to appease the ringleader never worked, and when we offered them an inch, they’d take a mile. They thought they were above everyone else and saw no need to cooperate. When we finally removed them from the community, it was astonishing how quickly the community improved with that one change. We found that there were a lot of community members that were not participating because of the environment the ringleader fostered. It took years for the ringleader to gradually move the line in the sand, but only several months for it to recover, despite how cultish the clique he developed was. That’s not to say that the clique improved. They just became silent after the example was set, and were later drowned out and became unwelcome when the community defined its new standard. Like Sam mentioned, genuine toxicity like this is infectious, and the sooner it’s dealt with, the better.
It’s important to not fall into the trap of assuming every vocal user is the same as your worst though. Some users are negative just because they’re passionate about the product and have been raised in a bad environment. The reason our community recovered so quickly is because after setting an example with the ringleader, we not only established the proper way to surface feedback, but made it much more effective than being nonconstructive about it. Naturally, genuine community members tend towards the more successful mechanisms for providing feedback. If we had been overly aggressive in our example or ignored community feedback afterwards, it probably would have stoked flames instead. Pick out the people you’re sure are acting in bad faith, make an example out of them, and then immediately get to work on addressing feedback from anyone who provides it constructively. It may be helpful to elicit feedback from non-top-contributors if the top contributors are too reluctant to provide feedback just to set an example that people who provide constructive feedback get listened to.