Cancel The Character To Be Written?

The follwing function is in an onkeypress event for some textboxes. How can
I cancel the keystroke? or more importantly it's output. This function does
what I want it to do, but it actually writes a space in the textbox. I
wanted to escape out of writing the character. Especially if I change it to
another keyCode besides space bar. Any ideas?

function setEfforts(ctrl){
if(event.shiftKey && event.keyCode == 32)
{
var row = ctrl.parentNode.parentNode;
var inputs = row.getElementsByTagName('input');
var textboxes = new Array();
for(var x = 0; x < inputs.length; x++)
if(inputs[x].type == 'text'){textboxes.push(inputs[x]);}

for(var y = 0; y < textboxes.length; y++)
textboxes[y].value = ctrl.value;
}
return false;

}