InfoPath Dev

I have two fields on a form. Category and Sub Category. Category is a drop-down list box and Sub Category is a multiple-selection list box that is filtered by the Category drop-down. Everything is working (mostly). The problem I'm having is that if I make a Category selection then make a couple of Sub Category choices and then change my mind to a different Category, the previous Sub Category selections are still shown in the list by their lookup IDs. See my attached image to see what's happening. It's easier to show than explain. I was thinking I could somehow put a rule on the Category field so that it resets or clears the Sub Category field whenever the Category field is changed. I know I can set a field's value but I can't figure out how to clear the Sub Category multiple-selection choices. Thank you for your help!

Multiple selects are very hard to work with using regular rules. If you add a rule that sets the field the multiselect is bound to to blank, you'll end up with a bunch of empty nodes - InfoPath doesn't provide a way to delete the nodes. I don't think there is a way to do this without code or a third party tool like qRules, I'm sorry - I know that isn't the answer you were hoping to hear.