@deepthisw, assuming that these are net new people who, then new people imported into Marketo do not get synced over to the SFDC on their own via the native sync (unlike those created in the CRM). You need to sync people from Marketo to SFDC for the first time by implicit or explicit means (check out Implied Salesforce Actions here). So, if you don't have any operational or local campaigns that would sync new people created in Marketo, then you should be fine, and these new people would not sync over to the SFDC. If you have an operational campaign that'd potentially sync people over to the SFDC, then you can exclude these people (i.e., suppress) by including a filter (e.g., Member of List not in <list name>) in the campaign's smart list.