I'm looking for English equivalent of Polish saying "Wpuścić chłopa do biura to atrament wypije" literally meaning "You let a boor into office and he will drink ink".
This saying is commonly used to describe uneducated, ignorant or even aggressive person that has no idea what he's doing or is doing something that is commonly punished by society. That person is also unaware of consequences of his activity or pretends to be and even if you make him realize that he will ignore it. You can also say that it is highly predictable that if you let him into some space/give him some tool he will do the most idiotic thing one could imagine.
- We're not allowing students to use lasers since the last accident when one of them looked straight into the beam without protective glasses and damaged his eye.
- My God, you let a boor into office and he will drink ink.