If you want to have the field updated in Marketo, but not in SFDC, you can prevent this with a SFDC validation rule. Below is an example rule that blocks an update to the Phone field if it already has a value and the update is being made by a user with the Marketo Synch profile.
When you make the change in Marketo and synch it to SFDC, the synch will fail with the error message specified in the validation rule above. Keep in mind that Marketo will still have the changed value unless you blocked field updates on that field in Marketo.