How to lock source code?

Hello everyone. Greetings to you all. I am a unity developer and a freelancer too. Basically i use unity to develop all my game apk. When you are in freelancing, sometimes you need to deliver your work to your client without being paid. That is the problem. All are not honest. I could be deceived by them, they can decompile source codes and leave me unpaid. So it would be really appreciated if someone could tell me how can i lock all the source codes that my client can not loot that unless i deliver him individually.

Thanks in advance.