It's not "marked as inactive", it's just a display telling you whether the workflow is active or inactive. (I think the phrasing is weak here, I'd prefer "workflow is in use / is unued" because that does not imply any flags or settings, it just tells you what's happening)
To get the workflow to be inactive, you have to stop using it. Look at the list of workflows, find the one you want to deactivate and you'll see a list of "workflow schemes" that it is in. Now move to the list of workflow schemes, and you will find that one or many of the workflow schemes your workflow is in have one or more projects next to them.
You will need to go into each project and change them over to another workflow scheme.
Your other option is to remove the workflow from any scheme (that is mapped to a project) where it is used.
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