I have created a custom list named “K2 Workflow Tutorials” in which K2 workflow will be created to stop inheriting site permissions. Please refer to my previous article to create a K2 workflow in SharePoint 2013 Online.
Go to K2 designer, drag and drop Stop Inheriting 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 “TEST SUBSITE”. After few minutes the workflow will be completed and the subsite which has the title field value “TEST SUBSITE” will stop inheriting site permissions.