Hiding Sales Stages

Hi there!

I am working on updating our instance from a very basic one to a more advanced. I am getting stuck in a few places, but today my fight is with the Opportunities module. I am hoping to be able to hide certain sales stages once a sales stage is selected. For instance, once an opportunity has made it to "Sent Agreement" I don't want our sales team to be able to choose the stage "Closed Other" only "Closed Won" or "Closed Lost"

Any insights? I tried in workflow, I tried in BPM, but I'm not seeing a chance to hide an option.

Thanks!!