Disabled users in Resource Groups come up in the scheduling assistant side panel on the Service Activity.
However, in the Resource selection page in the Resource Group Entity there are no views that show Disabled users. You also do not appear to be able to create new Resource Views.
Therefore you cannot remove a Disabled user from a Resource group.
To remove a Disabled User from the Resource Group and therefore from continually showing up on the Service Activity, you need to re-enable the user, remove them from the Resource Group then Re-disable them. It would be easiest if the disabled Users just didn't show up in the panel.