You will need the Full Write Workflows permission to perform this action.
Navigate to the workflow you want to delete and select "Actions > Delete" from the drop down menu in the top right.
You can also delete by navigating to the workflow listing and selecting "Delete" from the drop down menu at the end of the row containing the workflow you want to remove.
A pop up confirmation message will appear. Click on "Yes" to finish removing the workflow.
All processes and tasks will also be deleted
Any people who were currently in the workflow will also be removed. Their person profile will not be deleted however and history of what happened in the workflow will still be kept on their profile.