/include/class.ticket.php
Look for all instances of
$fields = array('type'=>'email', 'required'=>1, 'error'=>'Email is required');
and change the 1 to a 0. There are two of them if I remember correctly.
I do not advise doing this, as I do not know what other changes this action might necessitate, but if you really want to do it, enjoy. Note: I believe that email in the database is set to NOT NULL... so bare minimum that would have to be changed also. OST uses email addresses like a username for the person opening a ticket. Disabling it will probably break all kinds of things.