If you are S-On you may have to reflash the ROM yet again. Turn off the phone's data connection to stop an update, then use Titanium to see what version of the market/play store you currently have (to see whether it's done it already).
You need to disable the "market updater" app to stop this happening. If the market has not updated, use Titanium Backup to "freeze" the market updater. That should suffice. If it has updated, you'll need to flash the ROM again to undo that (no wipe, of course), then freeze it without letting it connect to the internet first.
The other way is to edit the zip before flashing, removing the market updater from /system/app, which will then mean that when you reflash the zip you will no longer have the market updater installed. Astro can edit zips on the phone (enter the zip, select the app, delete, back out of the zip. Then go in again and check it's gone, because it is sometimes tempermental), or there are utilities to do this on a computer (I'm not a Windows person, and can't remember what SUroot recommended for this).