C-g.
The STOP key will also abort the current
operation. This is useful if you are in the minibuffer answering a
prompt and change your mind. For example, when you press C--
(or C-x C-c) to exit, Emacs will prompt you if one of the buffers
has been modified since last saved. If you decide you don't want to
exit, you can just press STOP or C-g.