Can I create a class booking form which will stop accepting registration for a 9AM guest lecture and instead show the next available time slot? What would this logic look like? Thanks for your time - it's much appreciated.

Such a feature doesn't exist yet. It sounds similar to a "Garage sale" feature I coded a while ago, see #3 at www.limesurvey-consulting.com/integration/

You are such a help on this forum. Perhaps this feature is beyond the scope of LS anyway. Just thought I'd ask. Thanks very much.

Since being able to disable previously selected items is a missing feature, we have coded a Limesurvey plugin for this.
You can download the plugin from survey-consulting.com/product/limesurvey-plugin-garage-sale/ and test a demo survey at ls6plugins.mysurveyhosting.com/index.php...38?newtest=Y&lang=en