Western Advice: Sign an NDA or get a lawyer to draft up something with legal words that scare people to the bones.
Naija Advice: Sign the NDA, but understand that with the way our legal system is set-up you will find yourself fighting an endless battle if your code is actually stolen. It's not worth it, so don't put your faith in the NDA.
Instead ensure you find someone you can trust, someone that is recommended, don't pick someone with a shady background, pick someone with a track record, again this will not eliminate your issue but can help.
Also find ways to give people only segments of your code, sometimes they don't need access to the entire code to work on your site.
Last but not the least, stop worrying about replication so much so that it causes you to slow down your execution, you can buy an instagram clone or elance clone or facebook clone for less than $300 today. You could also find developers in India or other affordable countries that can replicate site functionality without skipping a beat. Point is that sites can be replicated, but what can't be replicated is your brand and the trust you have built with your customers and your passion. Put your effort there.
And please incorporate quickly and pay your taxes, those guys at FIRS are not playing in this recession.