How can I block signups by IP address?

[plasticsurgery]

I have a financial forum that receives an awful lot of spammers/scammers from countries in Africa and parts of Asia. In fact, 100% of members from many of these countries have fallen under this category, and they represent nearly 50% of new members. I had to change the settings to manually authorize members, but is there an easier way I can automatically disapprove signups from certain countries based on IP? I have no problem banning everyone from these countries if possible. Thanks