Here's my take on it...
Kids are going to try to explore boundaries. It's just how kids are. It's up to you to maintain the boundaries. You already knew that, of course. The question is... how?
I've found a few things key in my parenting:
1. Always speak nicely and respectfully, even if they're not, and even if you're mad. The more nicely and respectfully I speak, the more they are able to take away and learn for the future. Getting mad or yelling may solve it now, but won't lead to lasting life lessons.
2. Make the boundaries clear, and the consequences clear. Take time BEFORE the problems arise, to talk about boundaries and consequences in a nice manner, and talk about WHY those boundaries are there.
3. When they push the boundaries, be firm, consistent, and positive. "You know that XYZ isn't allowed, and that the consequence is ABC. Let's get through this and try to do better next time."
4. If they do something you haven't talked about, take it as a teaching time. "Hey, that's not something we should do, let's talk about why." Tell them why, tell them what the consequences will be, and tell them that they're expected to follow that advice in the future.
You're going to find yourself doing that over, and over, and over, and over, and OVER, to the point where you want to scream "DON'T YOU FREAKING GET IT?" But keep your cool, it's part of parenting. Kids aren't short adults, their brains aren't fully wired, and they don't think like we do.
Another thing that I've found GOLDEN is this: "I'd be happy to keep talking with you about this, when you're speaking as nicely as I am now. Until then, take some time to calm down in your room, and when you're ready to talk like I am, then come back out."
Good luck!