When you register on MyBB, you're asked to answer a question before you can complete registration. I remember that MyBB instituted this to prevent bots from registering. So set up a question/answer which only your target market would know the answer to. It needs to be easy and obvious.
Alternatively, you might want to look at plugins for your forum that require a Captcha to be answered and verifies email addresses during the registration process. If you want to take things even further, you might want to considering restricting the IP ranges of visitors to those only from South Africa. This would obviously only apply to your registration page, as you'd still want your forum accessible (i.e. readable) to anyone and everyone.
What you're experiencing is a problem on most forums from around the world. There are already solutions out there, you've just got to find them.