Unfortunately there are no really good preemptive measures against such attacks without compromising anonymity and/or decentralized approach. Involving mobile phones in the registration process however could make large scale attacks costly and less sustainable. If you tie the registration and application to IMEI or the serial of the mobile or maybe to both two then the attacker must have several phones and lots of active SIMs to be successfull. After that if you can make a low sell wall on one exchange for a few weeks or for a month as a second distribution phase (IPO, ICO, ITO, whatever) that will make the attacker's efforts much less profitable.