The only way to format the drive that is installed by default is the Disk Utility, or with the liveCD/USB but accidental formatting should not be possible. It cannot be done via Nautilus.
Disk Utility needs root privileges to make any changes so while you can get info on the disks and partitions, it cannot do anything else without a password. So if they have your administrator password the could reformat the drive but not with just a casual keystroke, they would have to
- select the drive and then
- select the option to reformat
- select the file-system to reformat to
- confirm and then enter your password....
When booting form the liveCD there is no password needed but to reformat they would first have to boot into the livedCD and then follow all the steps above minus the password. So it could be done intentionally but not accidentally.
If you don't want them to have the password you could set them up a limited user account and then they would not be able to use any
sudo commands, or setup your account to login without a password, (highly discouraged if this is a laptop in case of temporary loss). Slightly less secure if it's a desktop but anyone that has the computer in his possession and has a little know how can reset your password anyway....(can be done on Windows as well)
The fact that it is encrypted and by not be readable by Ubuntu makes no difference, the Disk Utility will behave the same.... the encrypted partition should just be listed as "unknown "
If you want you can remove the Disk Utility from the Software Center, and reinstall it if needed later; may be needed to erase a USB key or other drive but will not work on mounted drives so if you need to reformat you main drive you will need to use the liveCD/USB anyhow.