If there are awaiting email validation and not admin validation, you can configure the system to automatically delete those after a certain amount of time in ACP -> System -> Login & Registration -> Registration -> Remove unvalidated members.
The rest would need to be deleted. You can mass prune using the advanced search and then pruning members found. However, there is no criteria apart from the tabs to mass search for those users. Of course, with any mass deleting, take backups prior to actioning.