Hi Celia,
I believe you can query the Workflow table in a query to find all workflows with Status(state) equal to Started:
Query for all running workflows (query activity):
You may want to filter out any important workflows to ensure they continue running. Then use an Update Data activity to set the Workflow status to Finished (the value for finished is 20), this will set all workflows to a finished state, and they will stop running.
Update workflow state (update data activity):
Thanks