I have created a custom list named “K2 Workflow Tutorials” in which K2 workflow will be created to stop inheriting library 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 Library Permissions step from the Libraries Workflow Steps in the ribbon interface.
Specify a site collection, then click on Next button.
Specify filters for the library and then click Finish button.
Deploy the workflow. Create a new list item in “K2 Workflow Tutorials” list with title field value as “Documents”. After few minutes the workflow will be completed and document library which has the title as “Documents” will stop inheriting library permissions.