Owner Vs Creator
When you right click -> edit an object, and click "More" in the edit window, and select the "General" tab, you can see who created and who owns an object. There's even a handy link to their profile.
The Creator of an object is the original creator. This is very handy if you see something you like and want to buy a copy yourself. Check the creator, click their profile, look under their Picks or Classifieds tab, and teleport to their store. The creator is not the person to contact if an item, such as a boat, has been left on your property.
The Owner is the person in charge of the object. If the object bothers you for whatever reason, make sure you take it up with the owner and not the creator. You can IM them by going into the General tab in the edit window and clicking "Profile" to the right of their name. At the bottom of their profile is some buttons, including "Instant Message." You can use that to start up a conversation with them.
Please note that if they've left an item on your property, it's possible they crashed or lost it due to a physics issue and are not aware. Politeness can take you a lot further. Also, if the item is on your property, you can right click -> more -> return with no penalty. It will go placed in their Lost and Found folder. If the item is copyable, then you're safe to use right click -> more -> delete, too. Though in the case of deleting another user's object it serves the same functionality as return.
If you're renting and don't actually have return permission, you can have your landlord fix the problem, or contact the owner directly.