Change the Login Screen Power Management Preferences on Ubuntu

1. sudo chsh -s /bin/bash gdm
2. sudo passwd gdm
3. Switch User
4. Login as gdm
5. System > Preferences > Power Management
6. Edit as desired
7. Close
8. Logout
9. Login as original user
10. sudo chsh -s /bin/false gdm
11. sudo passwd -d gdm
12. sudo vipw -s
13. Replace empty password field with '*' character
14. Save the file

I used this, for example, to disable suspend on closing the lid of my laptop.