huh? Infrastructure is totally and utterly irrelevant to the problem. I know enough about commonsense to make such a claim.
Just send a new message, exactly as you would post an original item/comment/etc, but have some special text/field in there that says "please ignore the previous message". The UI would then hide the previous message.
eg
COMMENT: {id:9374758, from:"mibbitier", data:"I hate you all!"}
COMMENT: {id:9374759, from:"mibbitier", data:"*IGNORE_MESSAGE_IN_UI* 9374758"}
Nothing whatsoever to do with infrastructure. Nothing to do with caches. Purely to do with the UI. Not rocket science.
Granted, it's a poor way to do it, but it's better than nothing, and easier than trying to invalidate caches etc
Here's the thing that customers, managers, and less experienced developers all have in common: they understand that no one thing is difficult. But they don't take into account that managing the complexity between a thousand, or a hundred thousand, or a million rules is very, very difficult.
That's why you hire more experienced developers: they're more experienced, not at things like cache invalidation (sure, just nuke your entire cache anytime anything changes! easy!), but at managing complexity.
Which is difficult.
That's why I try to keep my mouth shut about how somebody should "just do this, it'd be so easy, why are they dumb?"