Ok just a follow up on the Sucuri situation.
Sucuri will scan your site with several different user-agents (including the one you mentioned).
During this scan, the bot will crawl through all links found in the site, and looks like it's hitting the registration link.
Sucuri will not follow the robots.txt directives. Since it tries to behave as a "real user" it'll ignore that file. The only way to do it would be creating a .htaccess file to redirect SiteCheck to a 404 or a different page when hitting those forms.
So you can add
tor back to the list if you wish, and add this to your htaccess file.
Code:
<Filesmatch "^(register)\.php$">
order allow,deny
allow from all
deny from 192.155.95.139
deny from 66.228.40.185
</Filesmatch>
That will stop Sucuri from hitting the registration trigger, and it is what they recommended to me.