Thanks for the update. It was helpful in order to understand the context. And I have been able to reproduce the issue.
In fact once you import a package without edit permissions the imported workflows cannot be edited nor they can be deleted. Even if the package is deleted (and select all the items to be deleted as well) the workflows would remain.
There is a workaround. You can delete the folder in which this workflow is part of. This would delete all of the containing workflows including the non-editable one.
If there are some other workflows in the folder you could export them in a package and import them after you have deleted the folder to restore them.