To remove Win10 preview from #WindowsUpdate after you ran PrepareWin81ForWindowsTechnicalPreview.exe is to remove KB2990214 *OR* KB3014460.

Also if you still have it after uninstalling those updates, delete

The “Signup” key in HKLMSOFTWAREMicrosoftWindowsCurrentVersionWindowsUpdateWindowsTechnicalPreview or the whole folder itself.