Unusual question - changing the name of the register.php file

[nerbert]

You might try editing in a hidden input in your registration form like

<input type="hidden" name="duck" value = "bird" />

then put a plugin at the beginning of register.php

PHP Code:

if($_REQUEST['duck'] == '') 
{
       die();
} 


I suspect bots have the register form ready to go and look only at the non-hidden inputs