It's easy enough to tell someone to never contact you again. But if you see them daily as you prepare to commute because you live next door, a lifetime of social pressure to not be so rude as to ignore people is likely to get them to slip up and at least say "good morning!", even if they want to respect your boundaries (and definitely if they don't, and are looking for an excuse to trample all over them while perhaps pretending otherwise!) If you share a common living space or other "shared business" it's even worse, as there's also the completely understandable pressure, if not outright need, to resolve problems involving that shared business.
Moving is an action that can help disentangle you from shared business - be it common living spaces, shared fences, shared neighborhood issues, etc. - and help dismantle habits that would undermine your boundaries.
Slaughtering the cows would be like... selling her car so her mom can’t use it. Her mom can’t hurt the cow anymore, sure, but there’s still no fence there.
In contrast, telling her mom “No, you can’t use my car” would be setting a boundary. But telling her mom “I don’t want you in my life” would also be setting a boundary. These are just two different fences. In both cases, the cow/car are just fine.
You can't solve a boundary issue by not dealing with the boundary issue. At best, you can avoid it and hope it's no longer a problem. The underlying issue still remains.