Adding this script to an open-end question will prevent this behavior for just that question:
<script>
$(document).ready(function(){
$('#[% QuestionName() %]').attr('autocomplete', 'off');
})
</script>
Alternatively, adding this script to your survey's HTML head tag will prevent this behavior throughout the survey:
<script>
$(document).ready(function(){
$('input').attr('autocomplete', 'off');
})
</script>