Magento: how to manually customer account activation

I'm d开发者_运维知识库eveloping a magento theme; but my customer asked me a particular modification: he wants to confirm manually the users registration. How can I achieve this? is there a plugin or I have to do this with an hacking the core code? Advice? thanks

The extension that clockworkgeek posted is great, but it isn't currently in MagentoConnect. So, here is the github link: https://github.com/Vinai/customer-activation

There is an extension for it:
http://www.magentocommerce.com/extension/489/customer-activation