Corrupt

Recently we had the contents of two gas cans stolen. We believe we know who it was and think it's likely they'll be back for more. I think a blown engine would an excellent lesson on why one shouldn't steal.

We've all heard the sugar in the gas tank trick, but does that actually work? They knew enough not to take the mixed gas (though they did take our mixing oil). If our suspicions are correct, they have a couple of quads and a dirt bike. Can anyone suggest an additive that will do plenty of damage but isn't likely to be detected until it's too late?