Prevent a program (steam) to start for a time period
How do you launch Steam?  I think you could create some kind of scheduled task in Automator/cron that either moves Steam.app to another location (preventing your Dock icon from working) or, if you use Spotlight, somehow breaking the application bundle.

For example, you could disable with:

mkdir ~/NoSteam/
mv /Applications/Steam.app/Contents ~/NoSteam/


And re-enable with:

mv ~/NoSteam/Contents/ /Applications/Steam.app


Kludgy but effective.