Is there a way to prevent the "submit" button on quiz slides from appearing until a learner has selected an answer?
Not by default with standard quiz questions.
But you don't really need to do this for the question to be effective. You can turn on the option that will show an 'Incomplete' caption (it's usually on by default for new questions). This will show a special type of feedback caption if the user tries to click the submit button before selecting anything.
If you want more control then you would need to create your own custom quiz questions using normal Captivate objects. Then you would be able to set something up that worked closer to what you're asking for. But it would not be a true quiz question and would therefore not have several other advantages that normal quiz questions have (e.g. Review Mode as an option.)
This was something my boss thought they had seen done on a video. I didn't think it was necessary since by default the learner can't progress until they answer anyway. I'm sure it's possible to hide a "next" button on a standard slide, assuming there are actions that need to be completed before the user can advance. Maybe that's what they saw.
I recently posted a couple of blogs about Quiz tweaking. In this one I show two workflows to 'hide' the Submit button until audio has finished to prevent that learner skips the audio by answering too quickly: