Not removing but creating a new List without that element with LINQ:
// remove
users = users.Where(u => u.userId != 123).ToList();
// new list
var modified = users.Where(u => u.userId == 123).ToList();
Not removing but creating a new List without that element with LINQ:
// remove
users = users.Where(u => u.userId != 123).ToList();
// new list
var modified = users.Where(u => u.userId == 123).ToList();