When you flag a post you can Take Action which should hide it straight away. An automated PM is then sent to that user to say their post has been hidden for {reason} and they can then edit the post to bring it inline with your site’s guidelines after a 10 minute window (built in as a cooldown). If they edit it it should reappear again.
If you want to send a PM about the post, pulling up the usercard on the moderated post and clicking Message should automatically include a link/onebox of the specific post.