Make Key Items That Cannot Be Used In The Field Not Show A Use or Register Option

In OpenContextMenu in item_menu.c:

                 break;
             case KEYITEMS_POCKET:
                 gBagMenu->contextMenuItemsPtr = gBagMenu->contextMenuItemsBuffer;
-                gBagMenu->contextMenuNumItems = ARRAY_COUNT(sContextMenuItems_KeyItemsPocket);
-                memcpy(&gBagMenu->contextMenuItemsBuffer, &sContextMenuItems_KeyItemsPocket, sizeof(sContextMenuItems_KeyItemsPocket));
+                if (ItemId_GetFieldFunc(gSpecialVar_ItemId) == ItemUseOutOfBattle_CannotUse){
+                    gBagMenu->contextMenuNumItems = ARRAY_COUNT(sContextMenuItems_Cancel);
+                    memcpy(&gBagMenu->contextMenuItemsBuffer, &sContextMenuItems_Cancel, sizeof(sContextMenuItems_Cancel));
+                }
+                else{
+                    gBagMenu->contextMenuNumItems = ARRAY_COUNT(sContextMenuItems_KeyItemsPocket);
+                    memcpy(&gBagMenu->contextMenuItemsBuffer, &sContextMenuItems_KeyItemsPocket, sizeof(sContextMenuItems_KeyItemsPocket));
+                }
                 if (gSaveBlock1Ptr->registeredItem == gSpecialVar_ItemId)
                     gBagMenu->contextMenuItemsBuffer[1] = ACTION_DESELECT;
                 if (gSpecialVar_ItemId == ITEM_MACH_BIKE || gSpecialVar_ItemId == ITEM_ACRO_BIKE)
                 {

Only Hide Register Option

If you still want to show the Use option, but just not show Register, change the occurrances of sContextMenuItems_Cancel to sContextMenuItems_KeyItemsPocket_NoReg and create that array:

static const u8 sContextMenuItems_QuizLady[] = {
    ACTION_CONFIRM_QUIZ_LADY, ACTION_CANCEL
};
+
+static const u8 sContextMenuItems_KeyItemsPocket_NoReg[] = {
+    ACTION_USE, ACTION_CANCEL
+};