I might be misunderstanding your question, but...

AXS doesn't have any settings that allow you to block any specific chain ring / sprocket combination.

However, it sounds like you're wanting to block the smallest sprocket / small chain ring combination. You don't have to "block" that combination, AXS doesn't allow it, period. That's the only combination that's blocked.

If you're on the small chain ring, the rear derailleur won't shift to the smallest sprocket in back. Just won't go there. You have to be on the big ring to go to the smallest sprocket.

If you're on the smallest sprocket in back and try to shift from the large chain ring to the small chain ring, the rear derailleur will automatically shift to the second smallest sprocket.

Does this answer your question? Sorry if I'm misunderstanding.