var 
 Flag : UINT; 
 AppSysMenu : THandle; 
begin 
 AppSysMenu := GetSystemMenu(Handle, False); 
 Flag := MF_GRAYED // Set Flag to MF_ENABLED to re-enable it 
 EnableMenuItem(AppSysMenu, SC_CLOSE,MF_BYCOMMAND or Flag); 
 
 //Or if you prefer, the following code will delete / remove it completely 
 {either 
 DeleteMenu(AppSysMenu,5,MF_BYPOSITION); 
  ALEBO 
 DeleteMenu(AppSysMenu,SC_CLOSE,MF_BYCOMMAND);} 
 {DrawMenuBar(AppSysMenu);} 
end;