Always respond to their concerns with professionalism, courtesy, and tact.
The absolute worst thing to do is to start censoring them. I've seen this happen with a number of GitHub projects in the past. People occasionally come along with ideas that some, or sometimes even just one, of the core contributors does not agree with. Even when the newcomers are participating in good faith, I've seen intolerant GitHub project leaders start deleting perfectly good comments, closing perfectly valid issues, rejecting perfectly good pull requests, deleting perfectly good wiki content, and otherwise engaging in behavior that is much more harmful than anything the newcomer could managed to have engaged in.
When I see that happening to new contributors, especially when it's a small project involved, then I know it's time to move on from that project, both as a user and contributor. I refuse to be associated with such nonsense.
Ultimately, most of these maintainers are volunteers doing what they're doing in their spare time out of love for their craft, and they have no social or professional obligation to let other people dictate the direction of their project. The nice thing about open source is that if there is a critical mass of users who are unsatisfied with the state of the world, they can fork it and take it in the direction that they want. Yes, "just fork it" is often used as a cliched excuse for shutting down conflicting viewpoints, but it's such an effective excuse because it's also a true statement.
What if there isn't a critical mass? What if there's an important project which has received and ignored perfectly reasonable user requests, or even patches that "just have to be merged", but there's no critical mass for forking? Sadly, that's how pretty much everything else in the world works as well. Like democracy and capitalism, it's the worst system in the world except for every other one. Everybody has a laundry list of complaints about their most-used software, and has thought to themselves, "if only they would change this one thing", "if only they would just merge my patch", "if only they would listen to me", then everything would be great. What the "if only's" of the world don't see is that for every popular project, there are dozens if not hundreds or thousands or millions of people who each have their own "if only" which in many cases are contradictory if not outright wrong. If someone just doesn't have time to work on their passion project and drink from that firehose at the same time, I do not judge them for choosing to focus on their own code.
It's very dangerous to give into that sort of pressure, for the next "request" might be very wrong. Although depending on the nature of the situation, that can be even more dangerous, since it has been demonstrated this can cost you your job or career.
(I'm afraid I'm come to the conclusion that I'm going to have to do future projects under a pseudonym, I have unPC discussion comments going back to the '80s that are still on-line last time I checked, all the way to this 10 days ago: https://news.ycombinator.com/item?id=9815052 .)
Too often I see people shut down and go into a kind of "you can't tell me what to do" coma and lash out. That doesn't solve anything.