Lauren, there isn't a global setting like this. Two solutions here:
1) Replace every campaign-level sync with a global sync that excludes customers. Depending on your set up, this might be based on lead score, recent activity, or just anyone who isn't a customer. Note also anyone in a Marketo Program will be synced to Salesforce if the Salesforce Campaign is synced to the Program.
2) Sync these customers to Salesforce and place them somewhere that won't impact things in Salesforce. This is far simpler to implement, and a more common choice. You might assign them to a specific lead owner (such as your Marketo Sync account), change their status to "customer", or anything else that will clearly communicate they shouldn't be touched by Sales.
Hope this helps.