freeing SetWindowLong() (windows)

After looking at the posts here: http://tinyurl.com/3nyugo
i managed to subclass various windows in order to get mousewheel messages, however i cannot work out how to free the subclassed routine when my object gets deleted or if the user just wants to stop the object running. Below are the two main functions and i have attached the full source.

Can any win32 programmers shed any light on this?

cheers,

oli

void mousewheel_start(t_mousewheel *x)
{
    void *window;

    if(x->oldproc == NULL)
    {
        window = jit_object_findregistered(x->ctxt);
        if(window) jit_object_method(window, gensym("get_window_ptr"), &x->hWnd);
        else x->hWnd = main_get_client(); //if no argument, get the patcher window

        if(SetProp(x->hWnd,"olmousewheel", x))
            x->oldproc = (WNDPROC) SetWindowLong(x->hWnd, GWL_WNDPROC,(LONG)MyOwnWndProc);
        else post("Couldn't setProp()");
    }
}

LRESULT CALLBACK MyOwnWndProc(HWND hwnd, UINT umsg, WPARAM wParam, LPARAM lParam)
{
    t_mousewheel *x = (t_mousewheel*) GetProp(hwnd,"olmousewheel");

    switch (umsg)
    {
        case WM_MOUSEWHEEL:
            outlet_int(x->p_out, GET_WHEEL_DELTA_WPARAM(wParam));
        return 0; // return, or can also pass to orig proc
    }

    if (x && x->oldproc) return CallWindowProc(x->oldproc, hwnd, umsg, wParam, lParam);
    else return DefMDIChildProc(hwnd, umsg, wParam, lParam);
}