[Clipper]

With those examples, no you can't stop them. The Epic Orc feat actually states "Your foes cannot play any cards to stop this" and automatic kills and defeats, such as a Janibot when there is a Gadgeteer present, happen instantly before anybody else can play any cards.

However, in both these examples, the combat hasn't ended yet, so you are still able to play a Wandering Monster into the combat. Once there is a new monster present, you could play Friendship Potion and this would stop them getting Treasures from the monster they instantly defeated as well, although they would still get the levels from that first monster.