Have reviewed your install. It's because you've so many plans available it's converting to a select2 automatically by CB. Selecting nothing should select the blank value now set as the placeholder value for the select2 usage, but seams that's not working right after something has been selected, saved, then you attempt to remove it after it has been saved.
Looks like this is a bug in CBSubs where hideblanktext is set on the XML node, which should not be set. Have added a bug ticket for CBSubs and will have a fix submitted to Beat for review today.
#6889