(I’m not advocating violence against worms. I like and protect them, they’re good for my garden. This question has been bugging me for a while and it doesn’t make sense. )

No, it's a myth.

Only one of the two halves would survives (the one with the head) but it's also possible that neither half would survive depending where the cut where made.

This does work for starfish though.

Almost. Some annelids really can regenerate from two halves, such as polychaetes. And actually, there is one type of earthworm that will evidently do this as well:

https://www.google.com/search?q=Perionyx+excavatus&udm=2

not an earthworm, but Flatworms. flatworms belong to platyhelminthes phylum, and earthworms are annelids.

the free-living flatworms can be cut in to pieces and sitll regenerate full bodies, but you cant cut it too small though. this is why some of them are invasive species, very hard to get rid of.

you definitely dont want flatworms in you gardens though, they hunt earthworms and are toxic, and invasive.

How do you stop them?

I remember waaay back at school, we had to perform some tasks on animals, one being live planarias. You could cut the head in half, and get a two headed worm, so I am not sure if cutting all along would regenerate on 2 full bodies though

Dissecting frogs, chickens, whatever, was not funny times to me for sure

Gun

https://xkcd.com/1217/

In all seriousness, I think changing the PH and moisture level would drive them away. It likely depends on where they are and what other life forms are present

Only if the cutting includes a section of the central plate.

Here's a relevant video on the topic of regeneration

https://youtu.be/n5U8M2JtAHk