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Yes, you do this by first including the all-important FormsPlus::Tag JS helper (link in the demo, download it to your own server and include in your page after Marketo's forms2.min.js script).
Then use a little CSS (to eliminate any FOUC) and a little JS, as in MktoForms2 :: Self-Hiding Checkbox.
In the demo, the field Rep_Age_c is hidden if it's checked, shown if it's unchecked.
Make sure you run anything like this by your legal team before pushing to production. Not allowing people to uncheck a previously checked box is questionable.