For most lift out there, there should be some way to cancel a command (floor button). Some of them uses double press, other just hold and it will cancel that button. But the current HDB Lift I have from EM (I think) took me a while to figure out. Basically, in the end, it is just a simple press and hold any 3 buttons and all commands are cancelled.
Well, some of you will tell me to go RTFM. IF only the manual is that easy to find. :)
Anyway, if your HDB Lift is the new generation one, this may also work for you. Don't need to endure those empty floors if you make a mistake and press the wrong one or some bloody kid press all the buttons.