Term Checklist Walker – Disable Parent Categories

Currently I have a structure where the top level categories are acting like landing pages, so they should have no posts directly assigned to them. What I would like to do is disable the top level terms and only allow the user to check sub-terms (children). I’ve found a walker which transforms checkboxes to radio buttons here but I don’t quite understand it enough to only change the top level parent, it keeps cascading. Was hoping somebody could break down what is going on in this walker with the main question being “How do I disable top level terms in the admin panel term metabox”