Missed edit window: I meant to say, the effects of pepper exposure will be painful but temporary, and I wouldn’t suggest something that was going to cause permanent harm. But peppers are kind of a nice touch, because the pain and burning that may result from someone unknowingly touching that stuff and then rubbing their eyes or touching their unmasked nose or lips should be, I imagine, not unlike the sensation of tear gas. Y’know, the “law and order” stuff that Trump is fond of sic’cing on protestors.
Also, the problem with something smelly like ammonia is that while it may keep vandals at bay, they may just think some other vandal beat them to it, and it was a Trumpee who made your sign smell. Where’s the fun in that? (Plus, depending on how far the smell radiates, you might have to smell it too. No fair.)