First, I suggest upgrading to the latest version of mojoPortal. Then, follow Crispin's advice. You can restrict your login page by IP through the IIS IP & Domain Restrictions tool. 

I would also suggest changing all admin passwords to ensure they are secure passwords. I personally like Correct Horse Battery Staple: xkcd-Style Password Generator for password generation. Then, make sure you have mojo set to hash your passwords. 

We also tend to use made up email addresses for sites we don't need to receive emails from. This makes it much more difficult to guess the login information. We just generate an alpha-numeric password, use a few of the characters for the user and the rest for the domain, slap .com on the end and that's the login. So 4p25NgFfrC6c becomes 4p25@NgFfrC6c.com. Whatever you do, your site login should not be the same email address that you share with the world.

Thanks,
Joe

Your IIS logs will show previous requests to your login page, and might show that these all originate from a certain IP address or agent, or a small range of these. In which case you could block them either using IIS or the mojoPortal Banned IP Addresses page (Administration Menu > Advanced Tools >  Banned IP Addresses). But in my experience these kinds of attack often come from distributed networks of thousands of IPs, so blocking is futile. You might still find they come from specific countries, in which case you might be able to block at firewall level (so long as you do not want genuine users from those countries).