How to completely remove everything and start over?

So I recently hired a developer who totaly botched the project and left me with $300 worth of work that doesnt work. How can i totally scratch everything and start over? I removed the "customized" theme but there still seems to be a lot of leftover garbage that needs removed. I would rather just remove everything down to the databases to make sure there is nothing in there that could cause problems later. I'll just reintall WPMU and start from nothing.

Can someone point me in the right direction? Never done this before.

Thanks all.