It’s not that the Zillow app is on your iPad, it’s that the Zillow site can tell you are using an iPad, and is advertizing the Zillow app at you.
Unfortunatley some of these ads, either accidently or intentionally, will automaticaly redirect you to the App Store. Report this behavior to the Zillow site, and they should get on the case of their ad provider.
In the mean time, cleaning out Safari’s cookies and data is the first thing to try. If that does not work it may be necessaty to get more drastic. As Agent1191 says, this will also clean out data for other sites; however all this should do is make you log in again to sites that normally remember you. You could also choose to visit the problem site in Private mode, which should prevent any cookies, and hopefuly stop the ads form executing as well; though you’ll have to log into the site every time you visit. I’m not sure about that last, but it’s worth trying.
Good luck.