You can only delete lists that have never been used in a sent or scheduled campaign. Make sure to review the search criteria before deleting a dynamic list that currently contains no contacts, as the list may include date-based criteria intended to populate contacts for a future scheduled campaign.
Although you cannot delete lists once you have used them on a campaign, you can hide any list that you no longer need to use. Hidden lists can remain in use on existing campaigns, but they are no longer available for addition to campaigns. Hiding lists removes them from the default view on the Lists page, but you can display them by selecting the Show Hidden checkbox. Click the Show link to restore a hidden list to the default view and make it available for selection on campaigns again.