I believe the had to be a level of reasonableness when deploying a deterrent. i.e. What is the impact of the deterent and what is the likelyhood of an innocent party getting involved/impacted?
This is a good example of someone getting fed up and putting up a deterrent without thinking it through properly and it resulting in tragic cicrumstances -
https://www.dailyrecord.co.uk/news/uk-world-news/two-men-charged-over-death-16532357
Setting a "trap" for burglars that gave them a whack in the head if they forced open a locked door - sounds fine to me.
Doing the same if the door is to a public building and unlocked but just has a "no entry" sign on it - definately not as any innocent party could accidently go through the door irrespective of signage (may not be able to read; may not be able to read the language it is written in; may be blind).
Setting up a shotgun to fire if that locked door is forced open - totally dispropotionate deterrent.
In the case of the spiked fuel, an innocent party is not going to go up to a fuel store thinking it was free to use. That just doesn't happen. The only people who are going to access the fuel legitimately is the owner and no one else will access it for legitimate reasons. If the farmer set explosives up so the tank blew up when the fuel started to drain, then THAT would be disproportionate to the crime as the risk to life would be great (you don't get a death sentence handed down by the courts for theft, do you.), but spiking the fuel to cause inconvenience and possibly damage to a theives engine by using stolen fuel? seems a perfectly reasonable deterrent and it seemed to work as no more fuel was stolen. I don't think a "please do not steal" sticker would have worked? There was no loss of life or any potential loss of life. (Now if he was getting brake fluid stolen say and he tampered with that to make it corrosive to eat away at brake pipes, that could be considered OTT, but causing an engine to run bad and then break ... a broken-down vehicle is not generally dangerous)
And actually the farmer has lost out also as whatever fuel was stolen or not stolen, all the spiked fuel is probably no use for anything?
There was mention in a post about someone being culpable if they had a gun and it was stolen and used to hurt/kill someone? Well this is why there is a requirement to keep them in a gun safe. If someone breaks into a house and then a gun safe and steals a gun, the owner took reasonable precuations and has - and should feel - no guilt in the matter. Guns can be very dangerous and so additional safeguards are obviously needed. that goes without saying I would have thought.