Do be careful. In the US (and I'd imagine must of Europe, but don't know) it is illegal to boobytrap. Which (again US) has been upheld up include putting knowing harmful ingredients in food you expected to get stolen. Spicy food you intend to eat yourself is fine, but spicy food you don't intend to eat yourself may go over the line.
Haha, fair. I do think there's a difference in putting ghost peppers in a hot sauce vs putting them in a tub of ice cream. And only one is those is a good prank. Especially if it turns out the theif has an allergy to your peak ingredient (or claims they do).
I'm not saying you will get in trouble, just be careful.
For someone to actually be harmed from biting into soap, they'd have to actually swallow it. I'd think nobody in their right mind would actually swallow soap, unless by chance they don't have a sense of taste and smell.