If the quiz has some sort of time-limit, then use the option to automatically close attempts once time expires.
If that is not the case, then, well ... nothing exists yet.
There really should be an option in the quiz reports. There, you can select some attempts, and either 'Delete' or 'Regrade'. There should also be a 'Submit unfinished attempts' there, but no-one has ever implemented it: MDL-24814.