To remove a leech in Ark you either need a friend to remove it by holding down E, or the button you use to open doors on console. You will get a menu that has the option to remove leech. If you are alone, you can build a campfire, or hop on a lit cooking pot. Just make sure not to stay too long, because you will lose a lot of health. To remove a leech from a tame in Ark Survival Evolved look at the animal and do the same as above and you will get an option to remove the leech.