I don’t know if any of you all are still looking for a solution but here it is.
The task: To force Webflow to actually remove the Slide after it has been made conditionally invisible.
The problem: Webflow is hiding the CMS content for the Slide but not the Slide architecture itself, thus the blank ghost slide.
The solution: Push a .remove() command directly to the Webflow js for whenever the conditional visibility is set to be hidden.
Why this is the best solution: No new collections, no complicated site architecture changes, just plug the following code right into the “Before Body Tag” section of your page and publish.
Super easy, and quick, even if you have a ton of collection items.
Hope this helps someone! 
<script>
Webflow.push(function() {
$(".w-condition-invisible").remove();
});
Webflow.ready();
</script>