I think trying to explain the "whys" can dilute the effectivness of the boundaries. It's your boundary and you don't have to give a reason or explaination. I like what Interrupted said, "I just made them and started enforcing them." The more you try to explain the more opportunity it gives them to tell you why you are wrong.
There is a behavior management system for kids called 1-2-3 Magic. The two primary rules of this system are "No talking. No emotion." Here's a brief explaination a pulled of the internet. I realize that our favorite addicts are not children (necessarily) but behavior management techniques can work on adults too. You probably should "count" him though
"Counting is simple and efficient, but you have to religiously follow the no-talking and no-emotion rules. That means that after you give a count as a warning ("That's 1."), you are not allowed to chatter away and explain, explain, explain! When done properly, good counting and well-timed parental silence forces children to take the responsibility for their own behavior. "