We are using similar process for our sales team. In our case, if sales team finds that lead is not valid, they will diminish the score to lower value but not zero with reason field updated. On Marketo end, we do verify if lead score is changed from SFDC, they will get into the separate nurturing path.
- Instead of lowering the lead score from SF itself, I would suggest to create a Lead Status custom picklist type field.
- Add picklist value something like - "identified to reset score".
- In your Lead Scoring Program create a trigger campaign which listens to data value changes for Lead Status field with value "Identified to Reset Score" with flow step to bring Lead Score=0, if need be you can also add notes from this campaign, if you want to create more robust structure to help sales team identify reasons of sudden change in Lead Score value.
This is one place where separating demographic score and behavior score comes handy. We usually reduce the behavior score to 0 and lead the demo score untouched in this situation. We do it automatically when the lead is set to a "Recycled" status by the sales.