The first thing I'd do is understand with your current setup how everyone is syncing from MKTO to SFDC. Is this controlled via the lifecycle? Do you have a lot of random sync and add to campaign flow steps throughout? Are you using program to SFDC campaign sync functionality?
If you don't want certain people to sync over (I'd imagine in the future you'd have this use case for more than just the one form), you'll want to hone in on your sync flows. Without all of the info I'd recommend:
Hope this helps!
Not a best approach but you can ask your SFDC team to create a custom file and a script in SFDC which will check if the record comes with particular field eg. "Do not Sync = true" then just ignore the record from Marketo API and in the Marketo form add this field as a hidden with pre-defined value as TRUE.
Still won't apply to only that form's data.
Trying to reinforce the fact that the answer to the question is still No in Marketo.