Well actually this is the default behavior! If the user doesn't type anything in the email field then he won't be stopped sending the comment. But if he types something in the email field, then we will check it if it is a valid looking email.
Use firebug to examine the other fields. You can also edit the template and remove everything you don't need, but the thing is that the email is expected in the js functions, so the best solution would be to hide it with css I guess.