Is this grounds for thread closure?

I’m not defending @DrDeth’s argument, although since you didn’t name them, or even list the thread you were disputing in your post here in the ATMB implies that you aren’t as sure as your actions as you may believe. Instead, I’ll refer to this post of yours.

My useful suggestion is that, moving forward, we just ignore the same old disingenuous talking points rather than turning this thread over to re-litigating the same old bullshit like a thousand threads before. Maybe if we starve the bad-faith debaters of oxygen they’ll eventually return to their own laughing-gas-filled bubbles.

So you feel all his points were in bad faith, and that any ‘old’ arguments, as decided by you did not merit discussion. You didn’t refute him, which is kind of the point of Great Debates, you dismissed them. OTHER posters did a credible job of arguing fair points, and DrDeth, did himself a disservice, for which he was noted. When you were noted, you didn’t come here to dispute it, but utterly disregarded the mods instruction, which he noted when he closed the thread. Because, as a simple example, when I tried to bring up other possible points you went straight to the comment above.
The OP for the thread was a condemnation of the NRA and interpretation of the 2nd amendment. It was an essay on why/how it’s wrong. So arguably you should have put it in IMHO. But you chose Great debates. When another poster (not DD) brought up the issue of mental health, other posters disagreed, and gave examples and details why. Your response was

In keeping with my stated position that it’s time to move the conversation forward, without those who refuse to engage in good faith if necessary, I’m going to try not to respond to this kind of comment, which is nothing but the same old ultimatum and precondition intended to prevent productive debate.

So your debate was to not debate. Not argue merits, not bring up possible solutions to your own question. Note, I’m not saying that there isn’t nuance to all of these arguments, and that several are tired, or simplified. But you neither disputed by fact other posters arguments, nor provided suggestions of your own, to your own thread. Which is why I suspect the mods provided their instruction and why they closed the thread when it wasn’t followed.

Personally, I think there is tons of room for a nuanced debated and made every good-faith effort I could to address my feelings on both sides of the gun spectrum, but dismissing one side completely is closer to bad faith than the arguments you were railing against. Not going to reargue the points here, as that is not the purpose of ATMB, but both sides were largely talking past each other while decrying the other side by the time the thread was closed, and that action was probably for the best.