Do not show in User Picker users who are not members of groups/Project Roles ("disabled")

User Picker shows all matching users, including people who are not members of project teams through groups or project roles.
JIRA does not provide functionality to disable user accounts (JRA-2220), so the only way to "disable" them is to remove from groups/project roles.
Unfortunately they can be seen in User Picker and can be selected by mistake.