You don't, probably.

As long as you don't have a chat window focused and accidentally press the enter key, you can just backspace out any accidental keypress that might happen.

If your notebook is nice and stiff, the pressure will be pretty evenly distributed across the whole keyboard. Keypresses should be rare with such low pressure per key.

Hardware solution: plastic tray over the keys.

Software solution: Keyboard Cleaner http://jan.prima.de/~jan/plok/archives/48-Keyboard-Cleaner.h...

Looks like a notebook. If its a reasonably thick/sturdy notebook and you don't push down extremely hard then it doesn't press the keys.

My laptop has a "lock keyboard" key on fn.