A quick post just to highlight a difference between Project Server 2013 on-premise and Project Online regarding being able to deselect “User can be assigned as a resource” from the Edit User page.
For Project Online this option is disabled as you can see below:
For Project Server 2013 (on-premise) this option is enabled:
For Project Server 2013 on premise where you can deselect this check box it is recommended that you don’t use this option as un-checking it can potentially cause issues in some scenarios hence this option being disabled in Project Online. I imagine that there will be a change for Project Server 2013 on-prem soon that will bring this inline with Project Online.