Empty Roll Up Value if There is no Match

[mvcmania]

Hi, thanks for this awesome tool, that is super handy.
My problem is , emptying the roll up value if there is no match.
I have a parent object which has User look up and im rolling up user look value to the parent User look up field when there is matching role is selected.
However if the role is changed or child record is deleted, i would like to empty the roill up value (user look up) on parent record. How could i achieve this?

Parent Object : Media__c Field : Manager__c
Child Object: Team_Member__c Field : User__c, Role__c

When a team member is created or updated as role is manager, i want to populate manager field on parent object but also i want to empty the value if there is no match. Appreciate in advance. Thank you.