According to their website:

A key part of Sia's approach to stopping Sybil attackers is proof-of-burn. Hosts burn coins by sending them to a provably unspendable address. Hosts are expected to burn a portion of their revenue (~4%) as a demonstration that they are real. Renters will select hosts that have burned coins with a probability that grows in a linear relationship to the total number of coins burned. Therefore, a host that has burned 2x as many coins will be twice as likely to be selected as another host that has all other factors the same. This provides a very important defence against Sybil attacks.

Is this something that deals with the problem you pointed out?

more on that here : https://forum.sia.tech/topic/108/how-sia-works