The way to go about prohibiting their usage altogether is specific to your installation method. But by default those languages are not installed into newly created databases and only a superuser can "CREATE EXTENSION" so without superuser intervention uses in the database will be unable to create functions in those languages. If that is an insufficient level of prevention you would have to somehow get your installation of PostgreSQL to not include those core extensions. I'm pretty sure that during compilation you can do this. For packaged distributions I do not know what all is involved (i.e., what you have to choose to not install).
Thanks David !
Yeah, the extensions for perl and python are not installed. Checked both pg_available_extensions & \dx output.
Can I run OS commands without these extensions?
Superuser: yes (specifically I'm thinking the COPY FROM PROGRAM command, but also C language functions)