子フォームのハンドルが取得できるなら
procedure DeleteButton(Handle: HWND);
var
Style, ExStyle: Cardinal;
begin
if Handle <> 0 then
begin
Style := GetWindowLong(Handle, GWL_STYLE);
ExStyle := GetWindowLong(Handle, GWL_EXSTYLE);
Style := Style and not (WS_MINIMIZEBOX or WS_MAXIMIZEBOX or WS_SYSMENU);
ExStyle := ExStyle and not WS_EX_CONTEXTHELP;
SetWindowLong(Handle, GWL_STYLE, Style);
SetWindowLong(Handle, GWL_EXSTYLE, ExStyle);
SetWindowPos(Handle, 0, 0, 0, 0, 0, SWP_FRAMECHANGED or SWP_NOMOVE or SWP_NOREPOSITION or SWP_NOSIZE or SWP_NOACTIVATE);
end;
end;