You would need to attach an event handler for onkeydown of your text box.
<input type="text" id="txtEmail" önkeydown="myFunction()" />
your javascript function can be like this
function myFunction()
{
if(event.keyCode == 8)
{
var result = "";
var emails = document.getElementById('txtEmail').value.split(",");
for(var i =0; i < emails.length - 1; i++)
{
result += emails[i] + ",";
}
document.getElementById('txtEmail').value = result;
}
}