Pretty simple.
Code:
WriteProcessMemory(hProcess, 0xADDRESS, 0x90, 1, NULL);
That would nop the code at 0xADDRESS (obviously not a real address, replace with the address of the code you want to nop).
hProcess is a handle (HANDLE hProcess = OpenProcess(PROCESS_ALL_ACCESS, FALSE,PROCESS_ID)) to the process of the game.
0x90 is the hex code for the nop instruction.
1 is the length of the code to write (nop = 1 byte).
NULL is given because we don't care how much code is written, supply this with a pointer to a DWORD if you want to know how many bytes were actually written (error checking, etc).
If you still don't understand, I think you should get a firmer grasp of the Win32 API.