Disable the post save process completely

Question

I need a way to disable the save process completely using an action/filter. Something that works (for e.g.:) in the query, posts_clauses or wp_insert_post/save_post/update_post hooks.

So far I only tried to return '';, which gives me tons of errors for missing values for post object parts in the admin UI.

This should happen "silently", so no errors get thrown when php_error/WP_DEBUG and such are set to TRUE/On.

Btw: I'm not asking for how to disable the autosave feature.