How do you NOT sync Apple TV purchases to iTunes?

This question is on behalf of my parents who have Apple TV - which i bought for them and i have become their tech support on 24 hour call ><

Background: My parents share the same account. Dad uses the PC computer with iTunes where my mother only uses Apple TV.

She purchases TV shows, Movies, Rentals, whatever all the time and my father was asking how to turn off the syncing so iTunes doesn't get filled up with all her purchases.

Dad has an iPad and Mum has an iPad. Both have iPhones as well. Everything on the one account. Mum watches TV shows on her iPad as well...but rarely syncs with PC computer. He said some of the purchases download, where others don't? And he is worried about mum's purchases filling up his computer memory!

So his question is: how do you STOP them syncing/downloading?

I've searched & googled but can only find questions from people asking how to sync...not the other way around.

Any ideas?