I've found myself too many times in quite complex situations. I am on a sever minding my own business when I get the hebie jibies when some random guy enters my house. I type:
"Hello there good fellow"
But he doesn't respond. Why? I do not know. But he wanders around my house opening chests, and of course I follow him (giving my all important task of crafting a sword) opening up the chests after him making sure he doesn't take my items. There are 2 situations which could happen and 2 things you can do for each action.
1. He steals
If he/she steals from your chest, well you have 2 option as well words or action? If there is an admin online i'de say words would be the best option (unless the admin is a 5 year old and has no clue what he's doing). Ask the admin,
"Admin, I have (insert player name here) stealing from me. Could you please take care of the situation?"
This is the best way to ask for help as screaming:
"DEAR LORD ADMIN IM BEING STOLEN FROM @!@#&@"
Is not as persuasive.
Lets say there isn't an admin online. What to you do? Well don't talk directly to him as it will just motivate him more. Action would be the best. If PvP is enabled kill him! If not.. well your only option is to salvage your most valuable items into your inventory. Leave your dirt to him.
2. He just wanders
If he just wanders around your home, doesn't steal, greif etc...Just let him be. No need to contact an admin or kill the guy. But lets say he does begin griefing. Well you would just have to follow the steps above. But if he is greifing action is the best way to stop him, especially if he's pulling out the tnt.

Thanks for reading,
WA1D0