Hello Peter !

If you want to leave only one category on the ads category page (page/create-ad) then you may add the RAW block to the Studio->Pages->Ads->New Add page:

<script>

  $("#bx_ads option[value='']").attr('value', '1');

  $("#bx_ads option[value!='1']").remove();

</script>

This block should be the last on the page.