Workflows can only be deleted if no Learners have completed that workflow.

In the image below, notice that there is NOT a red X next to the first workflow. This means that somebody has completed that workflow, and therefore cannot be deleted. However, the second workflow can be deleted.

To delete the second workflow, simply click on the red "X" and that workflow will be deleted.

In order to delete the first workflow (the one without the red "X"), you would have to first delete all history records associated with the first workflow.