That's the whole problem, and always unsolved (because it's hard). You need to be able to ensure that votes are made by real people, that votes aren't duplicated, and that votes are included in a count. Some of this is easy, some of this is near impossible. None of this is solved by a blockchain, which is at its core simply a remarkably inefficient, if decentralized, timestamping system. When a "blockchain" is presented as a solution, ask why the trustworthiness of timestamping was holding back a particular technology before now.
That's the whole problem, and always unsolved (because it's hard). You need to be able to ensure that votes are made by real people, that votes aren't duplicated, and that votes are included in a count. Some of this is easy, some of this is near impossible. None of this is solved by a blockchain, which is at its core simply a remarkably inefficient, if decentralized, timestamping system. When a "blockchain" is presented as a solution, ask why the trustworthiness of timestamping was holding back a particular technology before now.