Uncheck checkbox

It drives me crazy when the complicated stuff works but the simple stuff doesn't. After detecting that a checkbox was checked under the wrong conditions , I want to display an alert and then uncheck the box.  This javascript runs as part of the actions on the checkbox. Maybe someone out there can help me.

This occurs as part of the calculation on the checkbox

x=getField("SummerTuition").value
y=getField("AfterCare").value
app.alert("Value of Y is "+y)

if (LOTS OF IRRELEVANT TESTING && y=="Yes")

{
app.alert("You cannot have Extended Care with the "+x+ "Program") // Message appears
event.value="Off" This doesn't work.
}