If you need to stop all future actions for all targets in a current, active workflow, go to the Workflows main page and deactivate it by selecting the three dot Action Menu then selecting Deactivate Workflow on the drop-down menu, as shown below.
If you need to deactivate a workflow on a specific account while the account is in the workflow, you can do so either from the Workflow Log page of that particular Workflow...
...or from the Workflows section of the constituent's account page: