Robert - it's a bad idea to block the /ctl/ path - this is used for many things in DNN, including editing pages, and blocking this will break your site. If you want to change the standard registration page and block the default registration page from spammers, then I suggest you follow my Wiki article on replacing DNN registration page and blocking spam registrations. This shows how to do what you want to do without breaking your site.

To answer the other question - yes, doing something at this level changes it for all sites on the DNN install. So if you're going to block the default registration URL, then you need to reconfigure the registration page details for every site in the installation before you enable any type of request filter.

After executing and testing the procedure outlined at: http://www.dnnsoftware.com/wiki/page/...

I noticed that I had 17 spam registrations!

I then enabled captcha in the site settings->user account settings->Use CAPTCHA For Registration to see if this would stop it.  Sadly no!

I then re-enabled the request filtering and it stopped.

I have now modified the request filtering to:

<add sequence="ctl=register" />
<add sequence="/register" />

I'll let you know how it works out!  Even though the testing revealed that the /register and ?ctl=register strings returned an 404 rejection code the spammer was still getting through!  Must be another hole that is not affected by the:

Server Variable : HTTP_URL
Operation : Regex
Match Value : (?<=\?)ctl=register|(?<!\?)/register
Action : NotFound
Location : (empty)