How to make a form only once submittable?

What I achieved so far:
1) User A fills the form and submits
2) user_id is saved to the database
3) User A revisits the form, database indicates, that there is an entry already with the user_id. A message is displayed, that the user A is only allowed to fill the form once.
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Question: How to hide the form now or disable the submit button?
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TIA
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EDIT: Managed to add a custom code containing CSS. But is there a more elegant way?

Hi joachimliedtke,
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I think you can use an Event switcher in the On Load event. If the record exists show a message if not, show the Display Form action.
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Bob