The problem here is that you know it will harm somebody (even if it is morally justifiable, but let's work on the legal angle.) Therefore, you have a class of persons who are likely to be harmed by your actions, and as you are aware of their actions, you would be liable for any consequential harm. It is akin to occupier liability; the way I was taught was that if you believe you're going to get burgled, you can lock your house, but not set a booby trap. Another example is keeping guard dogs, but you would be going too far by underfeeding them so they become more aggressive. There was a case a few years back of a man who electrified his car to catch thieves. However, the courts held that an innocent person could have brushed past it and therefore that was an offence.
Of course, the above superlative suggestion of spicy sauce is completely legal and as owner, you are within your rights to do this (Assuming you like spicy food!) I would suggest Tobasco Sauce- thin, dissolves easily and is very very spicy...