>How to desactivate item from the main menu of my application ?

See SET SKIP OF in Help:

Enables or disables a menu, menu bar, menu title, or menu item for user-defined menus or the Microsoft Visual FoxPro system menu.

SET SKIP OF MENU MenuBarName1 lExpression1
-Or-

SET SKIP OF PAD MenuTitleName OF MenuBarName2 lExpression2
-Or-

SET SKIP OF POPUP MenuName1 lExpression3
-Or-

SET SKIP OF BAR nMenuItemNumber | SystemItemName OF MenuName2
lExpression4