Registrtion CAPTCHA for multisite?

Greetz!
We have had close to 500 registration requests from the domain hotmail.com since yesterday and all of them have spanish like names. This is very unusual traffic for us. I attempted to use the sabre.1.0 plug-in to put up a captcha but it blocked all attemps to register even with a correct captcha. Maybe because we’re on IIS and behind a reverse-proxy? At any rate I have had to block the domain in the registration settings section but that is only a short term measure until we can find a captcha that works in the network installation. has anyone found a captcha that works on windows in a network installtion?