The underlying problem is the lack of boundaries. Moving away does not solve that problem. Only setting and enforcing the boundaries will solve the problem.
No amount of distance is a boundary. People can travel, use cell phones, or any other method to get past distance. It is only a logistical complication.
Psychological and emotional boundaries are required. Cutting off contact by itself is not enough. You have to communicate that you are cutting off contact. That's what establishes the boundary. In many cases you won't get a restraining order if you haven't established that boundary. The restraining order is a tool for physically enforcing an established psychological and emotional boundary.
I agree with you, seeing how even my spouse's 16h-drive-away mother is still managing to insert herself into her child's life several times a day. I can see on my wife's face she's been talking to her mother when I get home. Even with new recent boundaries set (we're not visiting anymore, no more answering the phone or WhatsApp 33 times a day and then when not answering right away having other family call to argue that you're abandoning your family, fuck all of them), and I feel my spouse has made tremendous strides there, the mum is still finding ways to do this.
Moving away solves the problem if you move far enough away that they can only visit rarely. You don’t have to answer the phone and text messages can be easily blocked.
Until they show up at your door, decide to move wherever you moved, or even try to move in with you.
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.
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.
I don't think cutting them out of your life is setting a boundary. That's like the difference between putting a fence around a cow pasture (a boundary) and slaughtering the cows for meat. One is a bit more permanent than the other. Not really a boundary but an excising.