That one guy once straight up told my actor not to do what I said. IN FRONT OF EVERYONE. I told them to act a certain, and he quickly told them "no no no, do it like this instead"

I feel you man with this post. Much needed.

I am just starting out. So, it's mostly just me. But, thanks for the 'heads up'. I expect I will 'bump' into a few K-I-As on the way. I will try to get arround it with good pre-project planning, that way the crew will not be surprised when I shout 'No! My way!'

I learnt years ago that from the very start of any project to make it very clear to the crew that Make up does make up, Director Directs, Actors Act, Gaffers Gaff etc etc. and nothing else on set unless they have been given other positions in the shoot if its a small crew.
If they have time to advise others on how they should be doing their job ( unless it involves a sfety issue) then they are not paying attention to their own job. Even as a director I have to be careful not to tell my actors how to act etc etc, Unless I'm actually teaching them if inexperienced..
What I do do is ask everyone their opinion about a shot as sometimes I may have missed something plus being much older than most of my crew they see things from different angles. But still keep it clear the final decision is my choice as the director. This makes each member feel important and that they are contributing to the shoot no matter how inexperienced. We never look at any suggestion as silly as eventually it may turn out to be a very good idea later on in the shoot. This is different from someone shouting out what people should be doing in their opinion during the shoot.
I just recently had a young guy straight out of film school who was exactly as described in your story on a shoot I also was helping with their lighting. Apart from wanting to give advice to everyone he didn't even smile at any time. When I asked what the problem was as I like to make sure everyone is happy he said he was bored as he had worked on major shoots and expecteb a more professional team.. I later found out he had attended a shoot whilst doing his film course. The way he was speaking to the crew you would have thought he had been in the industry for years. Luckly on the second day of the shoot he didn't come . Which made for a much happier shoot and a funny thing happened. The shoot went very well even without his advice.

The Know-it-All can come out of know where. I was part of a small crew for a music video as the DP and I was responsible for setting up my own lights. I was asked by the director while I was setting up this one lights for this one shot"what I was doing" as if he did not understand. My response to him is I was being "P.R.O." To the best of my Knowledge I was trying to present the light to create the best exposure for the shot in question. So I had to go through the process of explaining what I was doing with the lights. This was something that I've identified as an individual that had very little to no experience with any other chair, other then director. The only projects he would work on where his own as the director. Never taking in the experiences of working the other positions on other films. I can assure you setting up 1.2k Arri film light, is much different then setting up a 500 watt work light. On a side note I do not work with this person any longer.

No one likes the know-it-all, but what do you do when you see a definite problem on set? My main function is ad an editor, but a few years back I was asked to help grip on a set. The director was new, and I noticed he was doing very few takes and very few angles on most of his shots. Also, he shot no cutaways or establishing shots. I held my tongue ( not my place) although I mentioned it to my friend who was on camera (who had directed much more than the director we were working for). He agreed with me but we both kept quiet. The final cut of the scene was not very good, no fluidity or continuity at all, and the rest of the project was not much better. Should we have said something?

Maybe in USA...
Not in Russia, absolutely.

FIre them from the start...period. No warnings, no conversation, no B.S. You'll gain the respect (and to a certain degree fear) from the crew which in the end will show you're a leader and not a negotiator. You're a Director, not a middle school counselor. Leaders lead.