Are these Email Validations or Admin Validations? If the former, please check that you have pruning setup, if you do not you can merely set this up for the desired days.
If the latter, you will want to perform an advanced search on the members screen in the ACP by clicking the cog wheel in the search box. Filter down your search then once the search results load, click the "Validating" tab. Before clicking the prune all found button in the blue bar, double/triple check you're on the "Validating" tab before proceeding.
For self-hosted customers, as always, before doing any large pruning, it would be advised to take a backup prior to attempting.