Do not allow Adding new items to Items Grid

We have a requirement from customer that,
in Purchase Receipt, whenever items are added to items table, using “Get Items From” → “Purchase Order”, we should not allow adding additional items to the grid.
I used the following code to restrict add rows to the grid.

frm.get_field('items').grid.cannot_add_rows = true;

It actually removes the “Add row” and “Add Multiple Rows” buttons, which is great.
But if I place the cursor in the last column of the last row and press TAB Key, it still creates an empty row. How to restrict this?
I cannot make the grid readonly, because the user should still be able to enter some details of the item