Assigning manually a score based on some statistics?
If you use a ML model to learn a score that you built based on your features the best thing that could happen is that your model learn exactly the rules you applied on your features. It would not be usefull as it would be equivalent to applying your rules in the first place. Supervised ML is only usefull to learn unknown rules.
Basically you know have two approaches :

Don't use ML and keep your rules as is. Even if it wouldn't be the most predictive model it might already be usefull to observe unusual behaviors.
If you want to go the ML way you need to build relevant target. Generally speaking that means having a solid, undiscutable definition of your target. In your case the target could be an action from the moderation. Then ML could help you find the behavior that leads to moderator intervention.