How do I stop ubuntu updates from taking over the MBR?

I have a perfectly wonderful multi-boot setup with a stand alone grub2 partition booting from the MBR and from that grub2 menu I can boot other operating systems on other partitions (using, for example, the configfile command).

This has been working perfectly till an ubuntu update came along today and overwrote the MBR, thus disabling my wonderful stand alone grub.

I got it back, but what I want is to never have ubuntu do that again. How do I make sure ubuntu keeps its grubby fingers off my MBR?