How to restrict an application to only one user

I'm creating an Android application for a fantasy league as part of a college project. It's a paid application, and requires each d开发者_开发百科ownload to be associated with only one team.

I'm confused as to how best go about it. If a user goes to settings and clears the data from there, all files, settings and databases would be deleted rendering a locally stored indicator for "The app has already run before and hence user is registered" as pointless.

Another option that had come to my mind was to store the IMEI no on a remote database (with SHA1 encryption to protect the data) but the problem with this is that, if a user loses or changes his phone, his further participation would be impossible. Additionally, users understandably don't like to share IMEI numbers and it's acceptable.

The best solution in my mind was to restrict usage of the app to the Google account that was used to download the application. With this, even a change of handsets would allow further participation of the user. However, I've not been able to find a way to obtain the Android Market account (only require E-mail) which resulted in the download.

So, my question is, how do I best go about with the problem that I face? Is there a way to get the Android Market username? Or is there a worthy alternative?

The server-side option is what you want, you can get a unique id from each phone and use that as your identifier.