None of those answers are working for me (either generating a 500 error or doing nothing). That is probably due to the fact that I'm working on a hosted server where I can't have access to Apache configuration.
But this worked for me :
RewriteRule ^.*\.php$ - [F,L]
This line will generate a 403 Forbidden error for any URL that ends with .php
and ends up in this subdirectory.
@Oussama lead me to the right direction here, thanks to him.