You cannot block it. You can temporarily disable it if you want to, but it is not recommended as long as you don't have an antivirus application installed on your system. If you choose to disable it, follow the steps below:
- Click the Start button, type "security" and click "Microsoft Security Essentials" or find the Security Essentials icon in your System Tray (usually represented by a little green house with a flag on top). Right-click it and choose Open.
- Click the "Settings" tab and click "Real-Time Protection".
- Uncheck the box next to "Turn On Real-Time Protection".
- Click "Save Changes" and you're done.
If you don't want to disable MSE, you can try adding your game to excluded files and locations by accessing Security Essentials -> Settings -> Excluded files and locations.