How to prevent managers from assigning goals once an employee's goal form is complete?

Dear Experts,

We have a goal plan in which we have enforced the sum of goal weights to 100%. However, our customer now requires that the manager should not be able to assign any goals to their direct reports once the employee has completed the goal-setting process and the sum of goal weights in the goal form becomes 100%.

Currently, the manager can assign goals by navigating to My Team > Career and Performance > Select 'Add Performance Goal' > Select the direct reports to whom they want to add a goal. But once the employee's goal form is complete, the manager should not be able to assign any further goals.