You will see an
icon under the post. Click on this, pick the most appropriate reason from the drop down list, and fill in any other details you might think necessary.
Using this rather than posting a pm to myself or another admin/moderator has the benefit of being seen by all the moderators so it should get dealt with quicker.
If you reply to a post you find offensive you just perpetuate the problem making it harder for admin to move or delete the post as is appropriate without deleting other related posts.
There is also a warning system in operation on here where the moderators can warn people that others consider their posts offensive, off topic etc, and after so many warnings an automatic ban can be applied, for a specified time. I can't imagine that we'll be needing to use it but it's there.
If you receive a warning please don't take it as a personal thing from the moderators, they will only usually come as a result of us receiving a report from another user.