There is already a pinned post stating this in the category (PSA: Be professional and selective when replying!). If a poster doesn’t see that, they certainly aren’t going to see this.
If there are legitimate questions about a job posting from people who are interested in it, it is correct and good that these appear on the topic as replies. This is so that OP only has to answer them once and other people can refer to them. Private messaging would be a poor solution here unless it’s a very specific question that wouldn’t apply to other people.
As for irrelevant replies, make sure to flag them and they’ll be taken care of.