Would you like the good news or the bad news?
Bad News:
You're not going to get an answer like "Oh, just call this function". If you really want to resolve this, it's going to get very involved.
Good News:
It's possible, and you're going to learn a lot about hooking, which is super fun!
Your question does leave a little to be desired to be able to give you a direct answer:
uses WinApi to load into my game
- how? CreateRemoteThread, SetWindowsHookEx, etc?
i can't find it anywhere
- what is it you're looking for, a loaded dll, code in the address space, a patched address table?
I realised it uses WinApi
- what's it using, how did you find out?
Without knowing exactly what it is your cheat/hack program is doing it's not possible to tell you how to protect against it. As there are many ways to skin a cat, the cheat/hack program could be doing any number of things to achieve its goal.
I would suggest this thread is a good start. People give some good methods for hooking programs, and also some good methods for preventing programs being hooked by them.
If you want to protect against this specific cheat/hack program, you will need to find out exactly what method it's using to hook your program before you can block it.
Have fun :)