Too violent for me! If you want to assert your boundaries, I'd just hang one that says "Do not disturb." That phrase is a classic.

[He raises his mug up from his lap and sips his tea. Then he adds, casually:]

Then there's this trick I've heard of, where you leave a sock on the doorknob to keep your roommates out. That's what some commoners use in place of a "Do not disturb" sign. Something about that always seemed a bit crass to me, though. Even if it's fresh out of the laundry, I'd rather not use a sock!