How can I wake my laptop from suspend only on power button?

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http://askubuntu.com – My Lenovo G560 wakes up from suspend in my backpack sometimes and I hate it. I would power off the hole system, but gnome shell (which I like) starts very very slow. Is there a way to wake from suspend only by pressing power button? I use Ubuntu 12.04 with GNOME 3.4.2. Thanks! (HowTos)