display menus on all page except one?

by

As I noted in my comment, you can use the negation operator, like so:

if ( ! is_page( 'phnom-penh' ) ) {
    get_sidebar( 'not-phnom-penh' );
} else {
    get_sidebar( 'phnom-penh' );
}

Note the ! before the is_page – that means NOT.

Also note that if you use more than once condition via || or OR and either of the conditions returns true – the sidebar will be returned.