I think I've found the cause: the action seems to have changed to "org.freedesktop.login1.reboot" (and the-like).
Adding the following lines as
[Disable lightdm PowerMgmt]
You still see a confirmation dialog but there are not buttons to confirm. Looks ugly, but works ;)
Unfortunately this applies to all users, not only the lightdm session, so you have to add a second rule to white-list them if desired.
Note that this method block solely reboot/etc commands issued from GUI. To block reboot/etc commands from command line one may use molly-guard - as explained in Disabling shutdown command for all users, even root - consequences?