Symptoms

How to delete SharePoint group provider from K2 workspace? There is only Add/Edit options available there.
 

Diagnoses

According to official KB "Important considerations when registering a SharePoint Group Provider in K2": "Once a SharePoint Group Provider label has been created for the site collection it should never be changed or removed.  When the label is changed or removed any existing running K2 Process Instances will break - as there will be dependencies on the label such as Destination Rules which are dependent on the label to resolve users."

The reason there is no delete options for group provider exposed anywhere in K2 GUI is because there is no easy way to check whether this group provider was used at some point in a process. As we cannot detect these dependencies we not exposing option to delete it (there is an open feature request to add this though). 
 

Resolution

In case you absolutely sure that group provider is no longer used anywhere you can delete it directly from K2 database.

BUT THIS IS NOT SUPPORTED OR RECOMMENDED FROM K2 SIDE

Scripts for deletion from database can be found below (use at your own risk and alway perform full backup of K2 database and create a copy of GroupProvider table before attempting this):

--This creates a copy of your original table for rollback purposes:

USE K2

SELECT *

INTO GroupProviderBackup

FROM GroupProvider

--Sample script for deleting entry from GroupProvider table

USE K2

DELETE FROM GroupProvider

WHERE Name = '%Name of Group Provider you want to delete%'