I came across a base in the Frontier of smp8. I checked the status of the owner of the base using the /p [username]
If it isn't yours don't touch it. If said player returns and finds you griefed it, you'll be banned. If a build is really old and derelict and in your way, I believe you can get staffs permission to remove it.
What wither said ^ If something is in your way, contact staff either in-game or on the forums. One of us will come out with you and take a look, and let you know whether it is something that can be removed. Due to every situation being different when it comes to player builds, there is no general rule for players. So, staff action is always required to determine the best way to handle the build(s) in question.
To add to what JD has said, even if you have an Established Outpost in the frontier you cannot just remove random buildings you want removed. You must ask staff to confirm the build can be removed. Also, any materials in that build must be stored in a chest for the owner should they return. It would be best if the chest was locked (with grief protection block at least) and a sign telling the owner to contact yourself or staff. This is only after staff have approved the removal of the build!
