How to tell your MIL not to spank your child??

Okay so I know this is a touchy topic so let me just say first.. I don't agree with spanking, spanking is not the only way to get your child to "respect" you, or teach them not to do something... I respect my parents and I was never spanked.

Now on to the question.. my MIL is big on spanking... I've seen her spank my husband's nephew for accidentally falling onto her face (he was 2 at the time)(and because he was smiling his teeth bit her on impact) because she was holding him the wrong way and he fell forward.

I just do not want my child being spanked, especially if my child "accidentally" does something. I really don't want to wait till I see her spank my daughter but I don't want to bring it up before the problem even arises... I don't know what to do. Anyone else deal with a parenting/grandparenting conflict like this?