There's no such thing as (completely) "prevent", just substantially reduce by making it more expensive.

Which makes me wonder how much these guys spent per phone number. If you figure you can use a phone number maybe 2 or 3 times per service before it gets blocked they could create literally millions of accounts with this operation across various services. I don't know what the black market value for a Gmail, Facebook, Instagram, Reddit, HN, etc... account is, but I suspect it is more than the price of a SIM card.