Final answer:
To prevent updates to the Lead Source field in Marketo from form fillouts, block field updates in the field management area of the admin settings.
Step-by-step explanation:
To prevent a Marketo form fillout from updating existing data in the Lead Source field, one can take the following action: Block field updates in the field management area of admin. This would typically involve setting specific field update rules that define when and if a field should be allowed to accept new data from form submissions. By configuring the Lead Source field to block updates, you prevent the system from overwriting existing data with new form fillouts.