You can report players on the server that are griefing other peoples stuff(or your stuff).

It's Preferred if you can get the full playername of the griefer, before reporting them here. Makes it easier for the admins to track down and deal with the griefers.

If you can't get the playername or don't know who did it you can direct an admin to the griefed area, where they can pull up a history of block edits in the area, catching the griefer. To direct an admin to the griefed area when your not on the server, you can create a private warp for them to use to get to the area, or you can post the coordinates and world name to the griefed area in (x, y, z) worldname if your running out of warps to use, you don't need to invite the admin to it as admins can use all warps, even private ones from other players, just put the warp name in the post when making the post.

For stuff other then griefing it would be good to get some sort of evidence of what the other player is doing like a screenshot.