When removing a Tech from the SedonaSchedule, the best way to do this so that you do not lose any history is by Inactivating the Employee Record instead of deleting them.
SedonaSetup
Go to SedonaSetup > Employees.
Select the Employee that is linked to the Tech.
Select “Inactivate” and Apply.

A message will open asking if you would like to remove that Employee from All Display Groups.

By selecting Yes, that Tech will no longer show on the SedonaSchedule and will automatically be inactivated under the Techs in SedonaSetup.
However, any Service Tickets/Jobs that were assigned to that Tech will still show that Tech and any information entered by them.