So I think you're talking about two different things.
1. SFDC field visibility in Marketo. Any SFDC field can be modified to be visible (or not) to the Marketo sync user. Be judicious about exposing fields to Marketo Sync User, as once you sync a field to Marketo you can't delete it, and extra fields can slow down your sync over time. Your SFDC Admin by default should keep new fields from being visible to Marketo Sync user, and only expose when you ask.
2. Blocking field updates in Marketo. Any Marketo field (whether synced to SFDC or not) can block updates from various sources—SFDC, API, form fill, flow step change data value, etc. This is for fields like Lead Source that you want to write once to and never overwrite. Or some fields you only want to overwrite in some instances.
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