While I agree a GM has final word on story and plot. I also do believe a GM has to be flexible enough to accept or at least hear out ideas of their members. Its the GMs job when to know to bend their rules or not to. I do agree that a GM sometimes has to be blunt and to the point, I am blunt and to the point that I was called "hostile" recently. But the member of the RP kept pushing and pushing their point of view. We gave them an answer the nice way the first time, but they continue going on about it. 

Happened on another website, where someone kept trying to give me shoddy excuse after shoddy excuse. Then wouldn't take responsibility for their shoddy excuse and tried to pin the blame on me. They uh also reported me too for "bullying". When it comes to being a GM I have a saying "It's policy, it's not personal" I have a policy. I have a standard. I am going to live up to that standard, but I will also be flexible enough to hear someone out. If, and that's the big word, IF they start off cordial, respectful, and word themselves properly.

"I disagree" is not the right word to use against a GM. Instead "I have some thoughts, do you mind listening". Sounds much better than "I disagree because [insert novel of why they think they are smarter or better than the GM or the plot of the RP]" 

Be Respectful. I am respectful to you. Don't. There's the door and I am going to come off like an asshole.