Hey Annie,
Here are a few ways you can accomplish this:
You can have your SFDC admin create a checkbox that once checked, you have a trigger campaign do just that.
Or, dependning on the comfort level of your reps with Sales Insight, you can create a trigger campaign with a call to SFDC. Then, in Sales Insight, in the dropdown that has "Add to Watchlist", there is an option to "Add to Marketo Campaign". Name the campaign something like "Zero out Lead Score", the rep will then add the Lead and the score will be zeroed out.
One last option is to add the Lead to a SFDC campaign, and upong syncing, you have the trigger campaign set up to zero out the scores.