If you have server access. You can configure IIS 8 to allow or deny specific query strings contained in the requested URL. For example, if a denied query string is found in a request URL, the request is denied. This option worked well for me. I denied the string ctl=Register.

To configure query strings by using the UI

1. Open IIS Manager and select the level where you want to configure request filtering.
2. In Features View, double-click Request Filtering.
3. Select the Query Strings tab, and click either Allow Query String or Deny Query String.
4. In the Query string box, type the query string.
5. Click OK.

Cheers,
Albert

Denying ctl=register in iis gets a 404 page on my site now, but if I use MySite/Register, that goes to my registration page. Is there something that can be done about that little hole?