I'm looking for the flag (in bash or ksh or zsh) that disables all
extensions above posix and make shell strictly posix-compliant
(without any additions).

I found '--posix' flag of bash, but it's something different:
it doesn't seem to disable bash-specific extensions. It
"Changes the behavior of bash where the default operation *differs*
from the POSIX". For example, the ((...)) construct is still
interpreted
with --posix.

So, is there "restrict to posix" flag to bash/ksg/zsh ? (I don't have
ash)

Thanks
Yakov