If Checkbox is checked then just uncheck

Basically, I want an undo.

If I check the box, a bunch of stuff happens.  But then I realize I checked the wrong box and want to uncheck it and return to the default state.  All help is welcome. Here's what I've tried:

//Whoops I didn't mean to check that box.  I want to uncheck it. (doesn't work)
if (this.getField("splyAD700").value!="On") {
this.getField("splyAD700").checkThisBox(0,event.target.value = "Off");
} else {
//This is what should happen when I check the box. (it works)
this.getField("splyAD700").checkThisBox(0,event.target.value = "On");
app.launchURL("https://www.somesite.com", true);
}