There are a few different types of users that can be known to behave like this. Some we can do something about, some we can't, and some we can but shouldn't.
The first type of user likely to exhibit this behaviour is the "next Einstein" user; one that is not well-studied in physics and has thought up their own "theory" concerning the issue. Often times, they will learn some idea from pop-science that is either wrong or misunderstood. The theory they create will be, to them at least, simple, intuitive, and obvious, yet still based on the incorrect physics ideas. When you confront them about it and give reasons why their way of thinking is either wrong or not the only way of thinking, they usually cite meaningless examples or ideas and claim you are missing the point of their genius. These users are easy to deal with and sometimes fun to engage. Often, their posts are non-mainstream and can be closed. Their answers don't usually attempt to answer the question and can be deleted. If they do have a valid attempt at an answer, it can be downvoted if wrong. Basically, treat their posts like any others and don't let it bother you that they won't be swayed. Next Einstein users will always be a problem, you just have to learn to live with them.
The next type of user likely to exhibit this behaviour is a technically proficient troll. These users know enough about the subject matter to be able to answer questions, but they choose to do so in ways that are deliberately misleading and/or obfuscating. Sadly, there's not much we can do about them. Do not engage these users. That way lies madness. A technically proficient troll knows how to work the system so that they don't get banned and their answers don't get deleted. Just ignore them if you can.
The next type of user is what I call a fringe physicist. These users are people that know the physics fully well, but they seem to strongly dislike adhering to mainstream viewpoints. This may be because they secretly cling to their own theories, which skews their perspective of the real physics, or perhaps because they are simply the physics version of hipsters. It doesn't really matter. These users are arguably the most annoying of the bunch because they will always find a way to argue with you about accepted physics and will always make their posts emphasize the fringe or less accepted theories (with the occasional reference to their own unsubstantiated viewpoints) and will always try to downplay or cast doubt on accepted theories. You can attempt to reason with these users, and that is their redeeming quality. Usually they will accept that you are trying to showcase the accepted mainstream viewpoints so long as you give the fringe ideas an honorable mention. But when you confront them about a post and tell them they should omit personal viewpoints or mention the more accepted theories, they will usually mention some unrelated details and complicate the issue. Sadly, there is not much that can be done here. If you disagree with how they present their answer, downvote it, that is all. I don't like it any more than you when someone that knows the physics focuses on fringe or non-mainstream ideas to explain a simple concept to someone. It's misleading and it makes physics look like it has no clue what it is doing (and I suspect some of them are trying to get exactly that message across). These users will likely accuse you of being dogmatic and of having a restrictive view of the world. Don't let it bother you. This site is primarily meant to showcase the accepted and mainstream views of physics, not explore or emphasize fringe ideas. They just don't seem to remember that fact most of the time. However, these users are still valuable members of the community because in a majority of cases they will present answers to questions where there are no fringe theories and, knowing their stuff, they will get it right. Fringe physicists are all around. You will have to deal with many of them and learn how to get along. Do not let them push you around and be vigilant to make sure they do not lead people too far astray. But in general, avoid them if you know they will be acting fringey and be friendly all the other times. These people have chosen a valid point of view, it's just a bit grating at times.
The last type of user I can think of is the active researcher; an actual physicist working on a paper or research of some kind in that area. For instance, if a physicist is working heavily on $f(R)$ theories or on massive gravity, they might be more likely to focus only on those alternatives to dark energy/dark matter. This is fine. They can provide valuable insight into alternative theories about a topic. Unlike the previous class of users, these ones have excellent reasons for wanting to focus solely on the alternatives to mainstream. They will likely not hesitate to briefly mention the mainstream theories if pressed, but it's probable that they are leaving it out of their posts so that someone more experienced with them can write about them. We absolutely like these users and even if we could do something about them, we shouldn't. Physicists working on not-yet-mainstream research should be encouraged but sometimes reminded that their work is not the only solution. They can forget that sometimes; it happens to the best of us.
In any case, however, treat a post like any other. If you don't agree with it, downvote it and leave a nice comment explaining what you dislike (DON'T engage the technically proficient troll!). If it's irrelevant or not a fit for where it was posted, flag it for close/deletion. If you explain yourself and make it clear they haven't included a mainstream viewpoint but they still insist their view is the only possible solution, ignore them. Unless you know how to deal with this situation, it's usually best to ignore it. Don't wear yourself out trying to convince people, just address their errors, downvote it, do what you can, then move on.
