How to disable Play store from updating itself?
For now, the only solution I've come up with is to use the really old 2.3.6 version of the market app. 

This is the only one that doesn't auto-update itself.

In order to install it, you have to uninstall the play store, install it, and convert it to a system app.

If you wish, you can use titanium backup to backup the old version for later use.