Support staff don’t normally appear under Focus>Person>Manage Classroom Staff
If they do then it is likely that they have either been used for Cover or had a timetable in the past.
It is not advisable to change the staff code as it affects the personnel record but schools can remove the assignability for the year which would make the person not show in Cover.
Go to Focus>Person>Manage Classroom Staff
Search for the person, highlight their name
At the top click on the red cross Remove>Remove assignability from selected year