How To : Disable Enter key in web forms
Include the following code in the <head> tag, or in the common.js file in your website(hoping that you call it in the master page or the relevant form).
function disableEnterKey()
{
if (window.event.keyCode == 13) window.event.keyCode = 0;
}
And in the form that you want to disable the Enter/Return key, add the following code.
<form id="form1" runat="server" onkeypress="disableEnterKey()" >
Its completely situation dependent, but I use it at the master-page level itself.