Yep Volker is right, You can hide the Old step behind a active step. If the old step is seen even after hiding at the back, Try using your own Workflow step icon, Use a big icon so that the old step disappears.
Rather than trying to hide our disabled workflow steps behind others, we move them way down to the far lower right corner of the workflow screen. (so far that you have to scroll down a bit to get there). So when a user views the graphical view of a workflow, they don't see them unless they were to scroll way down in the graphical view, which most user won't do.
If you are not able to delete the requests using that specific workflow then you have only option of migration to replace the workflow.Do all the changes as per requirement in any of your Development or Training Env and then migrate that fresh workflow to your target Environment.