Hi Hector, At the moment you still need to use the extensibility options to add/remove buttons for different tasks.
Manage my substitutes is used for central substitution maintenance across all tasks - so just hiding the button for one task doesn't help you.
Suggest you raise a question about this in the workflow forum to get some more detail on how substitution works in workflow. It's quite tricky to avoid substitution on only certain tasks if they all run in the same ABAP instance.
Rgds,
Jocelyn