Thank you so much for your feedback!
You can kill them! You just need to attack them a lot! Depending on how strong they are they have more health. Best is to level your strength a bit before you go killing! (small tip the orange once are quite weak you can test killing on them again after you leveled strength)
Which version did you play? (Because in the old download it is very very hard to kill them. It should be easier in the new download and in the browser version)