Yes, as Princess Lea said, I now put a slip of paper with a curse
inside my boxes saying something to the effect of:

"This letterbox is protected by a very strong curse. Anyone who
removes this box from its location will have bad things happen to
them, and they will suffer from the consequences."

I sure don't take this curse seriously, but there are a LOT of
superstitious people out there who will think twice before tampering
with or removing the box.

It's not foolproof, but if it works 10% of the time, it's worth it!

You know what they say: What goes around, comes around"... and the
rat that messed with Lea's box, will have to answer for it somehow in
the grand scheme of fate.

Paul in SF