Locking on/off buttons to avoid accidental shutdown

Dear All,

I sometimes find myself holding the phone (L640 DS) in the hand together with some bags like supermarket bags or whatever.
By holding it tightly (in order not to drop it of course) I end up shutting down the phone by accident because I sometimes long press the on/off button while holding it.
It shuts down even with the screen locked.
Is there a way to lock the on/off button so that this doesn't happen?

AFAIK this isn't possible :/ I tried adding a pin (and I really hate doing that) hoping it would prompt for the pin before shutting down, but it didn't