I did open up a support case too with Marketo. There response was "that customizing the functionality isn't available, unfortunately." The "Patch is to uncheck the "Publish to Sales Insight" on each email template in Marketo. By doing this the user will not have access to the Email templates in MSI in Salesforce to send an email.
From what I tested this works, but the option to select the function from the MSI widget(VF page) still exists.
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