Just about any command can be stopped in its tracks with the abort command: Ctrl-C.
If you haven't actually executed a command, you can just backspace and erase what you've typed, but if you've already executed it then you can do a Ctrl-C to stop it.
Ctrl-C isn't a magic wand and it can't undo things that aren't undoable, like a partially complete format command.
However, for things like the dir command that seem to go on forever or questions you're asked at the prompt that you don't know the answer to, the abort command is an excellent Command Prompt trick to know.