Why send someone a box of rocks?
A better question might be, why not?
It reminds me of the old Peanuts Halloween special, "It's the Great Pumpkin, Charlie Brown".
When Charlie Brown goes trick-or-treating, every house he goes to puts a rock in his bag instead of candy.
It was just his neighbors' way of saying FUCK YOU to him.
Keep that sentiment alive and send a 20+ pound box of ugly, dirt-covered rocks to someone you dislike and generally have no respect for whatsoever.
And if they're not "as dumb as a box of rocks", they might feel just as shitty as Charlie Brown.
Optional: If you like, we can also include a poorly hand-written note buried amongst the rocks telling them whatever you want. Your call.