"I am going to open all mails that come to this house, even if it's yours."

Make sure it's a lot. Not just a dash. Enough to make *camera zooms in* A RASH

If I were feeling particularly vengeful, I'll use itching powder instead. Glitter can be annoying, but it probably won't compare to the satisfaction to seeing the other party scratching themselves raw.

It's illegal to open mail addressed to someone else where I'm from....

Send bag of gummi dicks.... it's real.

Your solution in perfect. Both wildly annoying, poignant, and does not leave any permanent damage or physical harm... just a cheerful warning to but the hell out.

Glitter? Please, this is Insanity Wolf, not a carebear. Mail anthrax.