In version 1.5, when I click the X on a region, it disappears, but then reappears when I close and reopen the region menu. As far as I can tell I can't actually delete a region without it appearing again when the menu is reopened.
It looks like I actually can delete regions, but if the region name starts with a ".", the region cannot be deleted. I went into regions.json and removed it manually.
In version 1.5, when I click the X on a region, it disappears, but then reappears when I close and reopen the region menu. As far as I can tell I can't actually delete a region without it appearing again when the menu is reopened.