You cannot delete an email list that was used in an email marketing or survey campaign.
To determine if the list in question has been used in an email marketing campaign or survey go to Workspace>Email Marketing>Campaigns. You can select Actions>Export to Excel File. A column called "Lists" will be included on the spreadsheet. You can do the same in Workspace>Surveys/Forms>Surveys/Forms List.
If the email list is included in an email marketing campaign or survey, you won't be able to delete it.
If you delete the campaign or survey, then you can delete the list. Keep in mind that if you delete a campaign or survey, you will lose all historical data related to it. If you do not want to delete the campaign or survey you can edit the list to make it inactive.
Donation Page Settings - Subscribe Donor to Email List
Event Setup Level - Subscribe to Email List
Membership Setup Level - Subscribe to Email List