There comes a time in every Unix user's life where they need to enter control characters into their editor. Maybe you're trying to add some ASCII BEL characters to annoy would-be cat'ers, or add some null characters or something, but most likely you want to search and replace!
Simply typing the control character itself is insufficient. You need the escape control character to escape your control character. Consequently, this is CTRL+V.
So to remove all the pesky \^@ from your pristine file, the keystrokes would be:
: % s / CTRL v CTRL @ / / g
You'll notice the CTRL+V adds a caret, and the CTRL+@ add the @. This is also good for those \^M filled files that come out of script(1)