Review your campaigns and surveys:
If you go to Workspace>Email Marketing>Campaigns, you can select Actions>Export to Excel File; Lists will be a column on the spreadsheet. You can do the same in Workspace>Surveys/Forms>Surveys/Forms List.

If the list is included in an email 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, you will lose all historical data related to that campaign. If you do not want to delete the campaign or survey you can edit the list to make it inactive.