We discussed this request internally a few weeks back, and then I kept my eyes open with it in mind.
Here are, I believe, the key factors to consider when constructing such a post notice:
Responses to the question have already gone off the rails. All of the proposed notices here serve only to remind readers of things they should already be keeping in mind when responding to questions. Or responding to anyone, anywhere, ever. The only reason we'd need a special notice is that things have broken down to the point where an extra reminder is needed.
It's worth mentioning comments as well. In fact, comments seem to spin out of control a lot faster than answers in these scenarios, so I'm kinda inclined to focus on them more than answers... But realistically, soapboxing in either venue is a problem and we need to discourage both.
The notice needs to get to the point, fast. If folks are already inclined to ignore basic etiquette because they feel it's sooo important to jump on their soapbox FOR THE GOOD OF HUMANITY... There's already a good chance they're gonna ignore anything meant to discourage that. Subtle, passive, lengthy reminders and links to other pages aren't gonna cut it - we need to convince readers that soapboxing is going to be a waste of time in as few words as possible.
With that in mind... Here's what I've implemented:
Controversial Post — You may use comments ONLY to suggest improvements. You may use answers ONLY to provide a solution to the specific question asked above. Moderators will remove debates, arguments or opinions without notice.
The default duration for this notice is 1 week; moderators can set it to time out sooner or later as they see fit.