Answer 1

The locking mechanism, if you're writing it in the C programming language will depend on the OS you're writing it for.

In Linux kernel based OSes there are two options in doing so. (1) One option requires 'root' access whereas the other option (2) requires you to be the owner of the app you're going to lock.
One common aspect for both the above options is an understanding of SELinux concepts in order to set it properly. This is the eagle's eye view of the locking approach on Linux platform. Also you need to know the programming internals of the Linux system to write an effective locking app.

In the Windows environment you need to know the Windows low level system calls API (available with the Windows SDK) properly to write a locking application.

Also, note that it is possible to write a locking app loosely without these system specific knowledge. It's not actually locking the app but more of encrypting the app such that a person with the right 'key' can open it.

P.S: Last but not the least is that in both Linux and Windows you can use the built-in system policies via scripting to lock or disable certain apps depending on the user. This approach require admin or root level access.