Schedule a shutdown that can't be easily cancelled

I have limited willpower. I often sit in front of my laptop and think, "I'll just surf the net for half an hour then go to bed". Four hours later I am still wandering through the library of Babel that is Wikipedia, eyes glazed, drool drying on my chin.

shutdown -s -t 300 seemed like a nice, simple solution. But the problem is that, "I'll just finish reading this page...shutdown -a..." is all too easy.

Is there a way to schedule a shutdown such that it caot (easily) be cancelled?