Hello!
On your CE router, on the interface towards core router, do the following:
ip summary-address eigrp 100 0.0.0.0 0.0.0.0
* 100 from the line above is the eigrp autonomous system number is just for example; you shoul use yours.
In this way the default route will be injected only towards core routers and not in other direction.
Good luck!
Cheers,
Calin