That is an interesting observation, Beth. You seem to be right about your description of the Quiz behaviour. My guess is that it's intended as a "Knowledge Check" activity, giving learners the opportunity to explore several options instead of as an actual assessment. A quick workaround is to disable the radio buttons once the "Submit" button is clicked. To do this, you just need to add a "Script" section from the "Optional Properties" column (last column in the editor) and copy/paste the following line of code there:
$("#checkBtn").on("click", function(){$("input[type=radio]").attr('disabled', true);});
Of course, anyone with a little JavaScript knowledge will be able to just enable them back again while taking the quiz! But if you have elementary or middle school students then this solution should be fine. I believe a better solution would be for Xerte to have a "Number of attempts" optional property.