I was tempted to say you should provide a set of adverts providing reasons to convert to a paying customer - I think the general principle is sound, but you could perhaps be a little more inventive.
You could collect information about usage.
- Number of hours spent using the app
- Number of day's it's used.
- How many sunrises have passed since it was first installed.
Show the passing of time in interesting ways - provide variety, maybe use animation or graphical visualisations .. highlight how useful the application has been to the user.
... and then let them dismiss the message and get on with using the app.
You have an incentive to show adverts - you get paid / but you don't have an incentive to annoy your user. Show them your message once (on load) in a memorable way and then let them use your application to its full extent.
In this case, I don't think providing full functionality is counter-intuitive to obtaining more conversions. Very few people will want to use their smart phones on a sustained basis without using mobile internet - and those who occasionally do, will be provided with an indication of just how great the experience is without adverts.