Users attempts to delete Group (List > Relationships > Groups > Edit Group > Actions > Delete) returns: This record cannot be deleted because it has dependent records.
Any Group which has been used to Bulk Merge operation or Marketing Campaign cannot be removed anymore.
Only one option is to inactivate the respective Group by marking Inactive checkbox on the Group record itself.
Note: Bulk Merge history is stored in Documents > Mail Merge > Merge History. Appropriate Group is displayed in Group column.
Marketing Campaigns are saved under Lists > Marketing > Marketing Campaigns.