You are not allowed to delete operator in the system, so you need to disable them instead.
This is because they're attached to the data in the database, and it could cause issue with reporting. Additionally, this removes all audit records linked to that operator.
For that reason, the system doesn't allow the deletion of operators. Instead, to disable an operator:
Click Master System then select the Operator Details Menu.
Select Operator Maintenance then change the expired or access disabled to Yes.