There are already some invisible limitation to smurfs.
I remember a post by riot about a year ago, where they said they have ways of identifying smurfs quickly, to raise their Elo faster and get them back to their level.
Obviously, that does not prevent a fair amount of smurfs from ruining low elo players's games on a regular basis.
As a general rule, I think Riot frown upon smurfs, but can't really prevent it of punish it.
They never said how they detect smurfs, because that would help smurfs fool the system.
But I can imagine that you could look for a bunch of factors : Connect with the same IP than a higher level player + never connect at the same time than said player + high KDR in every game + same friends as the main = probable smurf.
But even if you have all that, you STILL can't be sure it's a smurf. What if 2 brothers share the same computer ?
Also, it's still pretty easy for smurfs to avoid this kind of detection. Ask a friend to connect with your smurf once of twice from another computer, and play a game in solo queue where you throw, to balance the games you carry with your firends ...
If someone REALLY want to keep a low elo account where they can stomp on lower players, it's pretty much impossible to prevent.