How do disable background downloads?
Xcode will not download automatically until you actually press the download button in the App Store. 

The only way I can think of minimising the chance of using the WiFi extensively is to disable the installing available updates in the background.

Go to App Store->Preferences->uncheck Download newly available updates in the background



You could uncheck the automatic check for updates all together, but this increases the risk of not getting latest security updates - what is not really recommended.

All these modifications mean that you would need manually download updates.