Remove an Installed EP. This can be a little difficult on Steam, but there is a method. Usually if you just untick the EP in the Properties for Sims 3, you will begin to receive an error message that there was an error during start-up and "please see the log". Helpful, right?
In order to properly uninstall the EP, you must also edit your registry. As always, editing the registry is Serious Business, and could completely hose your computer or your game. Always back up your registry and Sims saves before doing this.
To uninstall an EP from a Steam install of The Sims 3 (do not skip a step):
1. Shut down The Sims 3 completely.
2. In your Steam Library, right-click The Sims 3 and choose Properties.
3. In the Download tab, untick the EP you don't want to use any more then close the Properites window.
4. Create a System Restore Point (in Windows, right-click My Computer or your version thereof; select Properties; select System Protection; click "Create").
5. In Windows, use your "Find" charm or "Run" box to run the program regedit.exe.
6. In Registry Editor, go to File > Export and save the file to somewhere you can find it easily. This is in case something goes wrong.
7. In Registry Editor, navigate to your Sims 3 registry keys. This is normally found under:
HKEY_LOCAL_MACHINE > SOFTWARE > Wow6432Node > Sims(Steam)
8. There you will see a list of all EPs and SPs that you have installed. Right-click the EP you want to uninstall, and choose "Delete".
After this, your game should start normally.
Steam Guide by cruinne