This is related to another post but the situation is a little different. I had a rocky session recently with my players. If you want to know more about that: this question gives some background. I choose to break this up into two separate questions because the question I originally had wasn't specific enough (in my mind) for this board.
So during the combat that eventually happened because the Bard pissed off the party, the fishman priest cast Spirit Guardians. When I explained the mechanics of how the spell worked, this seemed to make everyone upset. I basically decided that the first time it was cast counted as the Monk entering the area for the first time. Then if the monk started his turn in that area, he would be hit with the effect again.
The Bard launched into calling bullshit. The Monk followed suit. Then it seemed like every player wanted to have their opinion heard on why something didn't seem to make sense. This divulged into analysis of the wording of the spell, which this post already does a better job of discussing.
But this lead to about 30-40 minutes of people not able to let it go that I judged something a certain way and that I had final say because I was the DM and that the mechanics as explained in the link above are how I interpret the spell's text. This eventually caused a lot of strife toward me and I considered giving up on the group during this time. It was a really tough session.
To the question:
How best to handle a situation where you have to flex your power as DM without everyone arguing or feeling resentful?