How to turn off ALL comments for posts, pages, etc already created?

I’m looking for a quick way (preferably without some nasty SQL query) to turn off all the comments on a large (10K+) number of posts where the comment flag is apparently on.

I’d really just like to turn off comments site-wide, if possible.

Ideally, it’d turn off comments site-wide, then sometime, when I want to fart around with making comments work, it’d be nice to turn them back on.

Any suggestions?