Wait, so you can't roll over & splat the enemy? They just don't get splatted or what? O.O I don't think I'm going to like it then, but we'll see!
You have to use the button to flick ink to kill them. If you try and roll over them it just... Doesn't work. I think if you happen to roll into them to multiple times is might kill them. I haven't tried it though.