esc esc esc :qa!<cr> (8 strokes)

This is the shortest I can think of right now, 3 esc to exit Ctrl-v, Ctrl-r and insert mode, then :qa! to quit even if there are splits or modified background buffers.

If ^C is considered one keystroke I'd replace all esc by ^C to prevent vim from hanging forever in the following starting situation: 999999999asometext^R^V<exit sequence>

With esc it would try to insert a lot of text after the last esc, which will make vim unresponsive for days at least.

Not sure if you want to count this one: alt+sysrq+k (1 stroke or 3 strokes depending on how you count)