There is nothing you can do against smurfs, and it is also a financial benefit for Riot because these people buy the skins again.
The only thing that would occur to me would be to limit a number of accounts (for example 3) to one computer. Either by IP address, MAC or by HWID.
But there are also external tools that allow you to change all this and it wouldn't useful at all.
You can't do anything against them, just accept that in this type of games there are and will always be smurfs.
DemCyclone ^^^ direct quote from the great and almighty sergio himself and hes definitely not wrong. sadly this is the case