Currently, Workflows as a whole cannot be deleted. However, there are other ways to ensure that a Workflow will no longer automatically run.
Disconnecting the Workflow from the forms it is linked to at Step 1:
You can go to Workflows > Workflow List > select edit for the workflow you wish to disable > deselect the checkboxes for the forms that it's linked to > select an inactive Form Template (as the Workflow will not save if there are no template selected) > Save at the bottom of the page.
At Step 2:
You can delete each of the Stages by selecting Delete beside the Stage titles.
This will reset to at least still keep the 'Created' stage (or whatever the initial stage in your workflow is called), but there will be no register ID, conditions, or field changes. So the workflow will not only be attached to an inactive form that isn't use, but will also have no rules or extra stages to push the form through to.