Is there an over-arching command that prevents a list uploaded into Marketo from syncing to Salesforce

The first thing you'll need to do is identify what's causing the leads to sync. If you have activated the campaign inspector from the treasure chest in Admin, you'll be able to quickly check all your active campaigns and find which have Sync actions.

Once you've identified those, you can add conditions to prevent a sync. For instance, I have a "Suppress From SFDC" list that I use. So in my Sync to SFDC campaign, there's a filter that is "Member of List = False" and I specify my suppression list.

Then, I always upload these new leads into that list first, then, if necessary, I'll upload them again into a different list. But as long as they go into the suppression list first, they won't sync.