I doubt there's any way you can get the App store to stop hard-selling you Pages 5, but maybe what I did – until I decided I liked Pages 5 – might help you.
You can have both your old iWork Pages '09 & the new Pages 5 installed on the same computer, as I have them. You don't need to partition.
Pages 5 does not install into the iWork folder that holds Pages 2009, it installs into the Applications folder.
You can have them both in the dock at the same time and I've even had them both open with different documents at the same time, side by side in the same desktop, no problem.
One way to stop the prompts would be: keep the iWork Pages as you have it, download the new Pages 5 & if you really can't bear to look at it, stick it in a folder of its own, maybe titled "do not open under penalty of death!" – I'm not mocking you, that's what I would do myself.
After that, you may have to update Pages 5 from time to time but you can keep using iWork Pages 2009.
As I said, I now use Pages 5 for most things. Occasionally I need my old iWork Pages 2009 for a particular document or two & it launches without a hitch, entirely workable.
To reiterate, I use both versions of Pages on the same computer without any problems for either of them.