You know the saying... you can lead a horse to water, but you can't make him drink!
Some just think with their crops and stuff it as fast as they can... most times, they're ok, just keep an eye out for issues...
Just stop giving the corn... leave just their regular food out... they might pout and refuse to eat for a day or so, but they won't starve themselves if there is a food source available... I woukd continue to withhold the corn until they get comfortable with their regular, balanced diet... then just offer a handful once in a while, should make them extra friendly for you too...