Which one are you going to cull? The robber, or the one letting her get away with it?
Shoot her in the face with some pepper spray (bear spray) when you catch her. I have seen that work well enough.
I just missed a picture one year of two Angus heifers sucking each other after weaning their calves.
I had a couple nice captions picked out, and was bummed to have missed it.
Badlands