I have created a custom list named “K2 Workflow Tutorials” in which K2 workflow will be created to delete unique site permissions. Please refer to my previous article to create a K2 workflow in SharePoint 2013 Online.
Go to K2 designer, drag and drop Delete Unique Site Permissions step from the Sites workflow steps in the ribbon interface.
Select a site collection and then click Next button.
Specify filters for the subsite and then click Finish button.
Deploy the workflow. Create a new list item in “K2 Workflow Tutorials” list with title field value as “TEST SUBSITE”. After few minutes the workflow will be completed and the subsite which has title field value “TEST SUBSITE” will delete unique site permissions.