Why don't you and your BF make some ground rules? You can have set consequences for specific rule-breaking and that way everyone knows what to do and what to expect. You will not have peace until you work something out with BF and get on the same page. You could be the SuperNanny and it wouldn't work. He has to be on board and support you and the rules or it will just get worse and worse as she gets older and bad patterns become established.
You need to have rules about respecting other people's property, obviously and rules about respecting people and being thoughtful and polite. If she takes your stuff that would be breaking a major rule and you could give her a pre-set consequence that she knows about ahead of time (when you explain the rules and their consequences-or even post them on the wall, I did that with my skids and it worked really well).

We are really big fans of the loss of privileges consequence and this can be adapted to fit any kid. Some kids don't care about losing tv, computer or video game time but do care about time spent with friends. Anything they enjoy that is a privilege can be taken away if they break a rule. It sounds harsh, but is very fair. As long as they know all the rules ahead of time and what the consequences will be, it couldn't be fairer. The trick is having your BF equally committed to solid disciplining and having your back. You guys need to be a united front and discuss and agree on all of this privately and not in front of the kids. Then after that's done, you both need to be the enforcers so that one person isn't the bad guy all the time. If you are both consistent and don't allow exceptions this method of discipline works really well, even with kids that are nightmarishly bratty and rude (I would know).

Good luck with this, nothing would piss me off more than someone taking my stuff without asking!

"If you want to tell people the truth, make them laugh, otherwise they will kill you." ~Oscar Wilde