Gnomes keep exploding my toilets

Hi.

I have been playing Enchanted by Nature and I keep getting the gnome ailment which spawns repair/upgrade gnomes. These little guys are great- but they keep blowing up my toilet because I also have the Eco Lifestyle pack and they keep switching it to biofuel behind my back. I've had 2 sims die from flaming poop and it is incredibly irritating to throw away and buy a new toilet every time they come around. 

Please, is there any way to stop them from "repairing"  a bomb into my house? Or a way to remove that upgrade specifically since I don't want it and its explosive problems?

Help.