how to terminate a process created by CreateProcess()?

A handle to the process is returned in the PROCESS_INFORMATION structure, pi variable.

The TerminateProcess() function can be used to terminate the process. However, you should consider why you need to kill the process and why a graceful shutdown is not possible.

Note you need to set the cb member of si before calling CreateProcess():

si.cb = sizeof(STARTUPINFO);

EDIT:

To suppress the console window specify CREATE_NO_WINDOW, as the creation flag (the sixth argument) in the CreateProcess() call.

EDIT (2):

To suppress the window try setting following members of STARTUPINFO structure prior to calling CreateProcess():

STARTUPINFO si = {0};
si.cb          = sizeof(STARTUPINFO);
si.dwFlags     = STARTF_USESHOWWINDOW;
si.wShowWindow = FALSE;

In the struct pi you get:

typedef struct _PROCESS_INFORMATION {
    HANDLE hProcess;
    HANDLE hThread;
    DWORD  dwProcessId;
    DWORD  dwThreadId;
} PROCESS_INFORMATION, *LPPROCESS_INFORMATION;

The first parameter is the handle to the process.

You can use that handle to end the process:

BOOL WINAPI TerminateProcess(
    __in  HANDLE hProcess,
    __in  UINT uExitCode
);

hProcess [in]
A handle to the process to be terminated.

The handle must have the PROCESS_TERMINATE access right. For more information, see Process Security and Access Rights.

uExitCode [in]
The exit code to be used by the process and threads terminated as a result of this call. Use the GetExitCodeProcess function to retrieve a process's exit value. Use the GetExitCodeThread function to retrieve a thread's exit value.