Can’t erase spam messages

I’ve got a ridiculous number of spam comments (890), maybe a few legit ones thrown in. I want erase all of them. Problem is that the page lists like a little over 500 of them, then it gets to the bottom of the page and there are no options to select to erase them all or mark them all as spam. It’s like that bottom part of the page is missing.

Under each individual post there are the selections to “erase” or “mark as spam” but there’s no way I can do hundreds of them individually.

How do I clean this out, first of all?

Second of all, where can I get a plugin to stop this madness? Perhaps something that makes you type in some strange looking text.