Hi
Add the following in the Script-field (Right click form -> Edit form -> Behavior Tab):
function cancelEnter(evt) {
var evt = (evt) ? evt : ((event) ? event : null);
var node = (evt.target) ? evt.target : ((evt.srcElement) ? evt.srcElement : null);
if ((evt.keyCode == 13) && (node.type=="text")) {return false;}
}
document.onkeypress = cancelEnter;
A little explanation :
The script will cancel all 'Enter's (evt.keyCode == 13) whilst in a textbox (node.type=="text").