If you turn on team damage to kill them dont forget to turn it off again afterwards to prevent accidents. Personally I usually use team damage rather than the butcher knife so I can be more precise with what I kill when desirable. (Ceases to be an issue if you use autobreeders since your breeding stock isnt mixed in with what youre harvesting).

Use a butcher knife, I think its the only way

Or turn pvp on (the 2 swords in tab menu on right)

Nice!!!^^