Remove an user from item level permissions using JSON.

To delete a user from item level permissions using JSOM , use the following code.

var oUser= clientContext.get_web().ensureUser('your name@yourcompany.com');

oListItem.get_roleAssignments().getByPrincipal(oUser).deleteObject(); // oListItem- List item reference.

clientContext.load(oListItem);

Happy coding !!!