If you want to suppress the squid version in generated pages, add:
in squid conf
Disabling the error pages is not possible. What you can do is to change them to your liking
First of all add in squid.conf
to keep the original error pages intact.
Then copy everything from /usr/share/squid/errors/English (or /usr/share/squid/errors/en) to the directory specified above.
After this you can either replace the contents of every ERR_* file with your own message, or you can remove (using sed) the reverences to %s (the squid version)