trigger CommunityUserContactRemove on User (after update) {

    Profile p = [SELECT Id FROM Profile WHERE Name = 'Quatro Community Login User'];

    List<Contact> userContacts = new List<Contact>();

    for (User u : trigger.new) {
        User oldUser = Trigger.oldMap.get(u.Id);
        if (string.isBlank(u.ContactId) & u.ProfileId == p.Id){
            userContacts.add(new Contact(
                    Id = oldUser.ContactId,
                    Support_Community_User__c = null));
        }
    }

    update userContacts;
}