Ignore, or, if possible, flag
When someone says something rude, or points out how you're wrong bluntly, it can be pretty hard to resist that feeling to retort and argue with them. There's rarely anything to be gained from an argument, though, whether on a technical point or just arguing over whether their response was rude.
You can't win an argument, because if you lose, you lose it; and if you win it, you lose it. Why? You will feel fine. But what about him? You have made him feel inferior, you hurt his pride, insult his intelligence, his judgment, and his self-respect, and he'll resent your triumph. That will make him strike back, but it will never make him want to change his mind. A man convinced against his will is of the same opinion still.
— Dale Carnegie, How to Win Friends and Influence People
It's likely to be challenging to confront someone who's left a blunt, or even rude, comment. Most forums, though, have systems to discourage rude or offensive posts. For example, Stack Exchange sites have a Be Nice policy, which discourages rudeness and belittling language. The vast majority of other forums have similar systems to flag posts, although the level of civility expected may vary (some may tolerate posts like the one you mention, and some may not).
If it bothers you, flag and let a moderator handle the situation. They've been given the responsibility, and are likely to be in good standing in the community, so their words probably carry a lot of weight.
If the community you're in doesn't see a post like this as a problem, then your only real choice is to ignore it, or take it in your stride and respond as if it had been written politely.
For messages that are a little blunt, but not outright rude, it's up to you how you proceed. If it really bothers you and is hurtful, it seems logical to flag and escalate it—after all, a community that doesn't respond when users are uncomfortable perhaps isn't one worth being in. But if it's just trivial, either ignore it and move on, or, if you're willing, respond positively.
Debrett's also publishes an etiquette guide on How To Respond To Rudeness, weighing the pros and cons. As they state, responding in kind is great for letting off steam, but it doesn't really help a lot.