The fields and records that will sync from Salesforce are defined by the permissions of the Marketo sync user. Therefore, if you want to prevent specific records or fields from syncing with Marketo, change the permissions of the sync user so they do not have access to the records and fields you do want to sync.
Alternatively, you can work with Marketo Support to set up a Custom Sync Filter that will prevent records from syncing based on the value in a specified field.
Restricting Leads from Syncing to SFDC With Sync Filters
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