I've found the best way to kill
su during entering of an incorrect password is Control+U followed by Control+D. This should avoid annoying delays. You can also just clear the input with Control+U and re-try typing the password in.
Control+U is a command that clears the current line of text, and also works in a shell (and probably other places too). It's also nice that it seems to work with
su to clear the currently-entered password.
Using Control+C doesn't seem to work, it appears that for some reason
su just ignores it. It is untrue that the user who started the process can no longer kill it because of setuid --
su dies perfectly fine if you kill it with
SIGQUIT from another terminal (as a user that started it).
SIGINT doesn't work, so
su probably just ignores it.
Control+D with an empty password input seems to be the only way to instantly kill a running
su - if any text is entered, then hitting Control+D will induce a 3-second delay (unless you've actually typed in the correct password, in which case... it'll start a root shell).