What about First Name/Last Name? Can you also block those from syncing if data is already present in Dynamics? We're having an issue where people are typing their names into forms incorrectly and it is overwriting data in the CRM. If it can be done, can you do it from field management, or do you have to get Marketo support?
You can block field updates for those fields in the same way in Field Management. You would only want to allow updates from your CRM in that case.