If an administrator's account is restricted from making reassignments, that administrator can still make new assignments (just the reassignment feature is disabled).
You must have a system administrator account. If you are not sure what type of account you have, find out here.
Select "View Accounts"
Select "Administrators" for the account type.
Search for the name or email of the person or find their name in the list and select their name in the list below.
Select "Account Details".
Scroll down to the section labeled "Feature Access". Toggle the Enabled toggle to Disabled for Reassignments. This will prevent the Administrator from making reassignments.