Disallow execution of shell commands from psql

[Wiwwo Staff <wiwwo@xxxxxxxxx>]

Hi!
Happy new (gregorian calendar) year!

Somehow related to the proposal of having a `psql --idle` option, is there a way to disallow the command `\!` (and anything of the likes in psql?
 
Sure, I can set the SHELL env var at run-time, but I still want to have postgres user to be a normal user, with its shell etc, which means it can change this SHELL setting somewhere.

Any tip?