There are already SOME measures in place to stop people from making new accounts but those barriers can be easily overcome if you know a couple of tricks.
They need to implement a no multi-account rule that monitors transfers and account interactions. Clear and blatant abuses like the ones above should merit a lifetime IP ban from the site as well as confiscation of funds stolen.
That would be a massive invasion of privacy though. It wouldn't fly in the EU, especially considering that IP addresses are often treated as private information (as are phone numbers, for instance).
That's just a lawsuit waiting to happen.
It also defeats the purpose of having a decentralized social network. All of these things come with the territory. There are negative aspects. They are definite cons, but to come up with a centralized solution to these problems is also getting rid of everything that makes Steem better than other social networks.
IP addresses can be spoofed too.
Many people have multiple accounts on purpose to run bots. Some bots are very useful. Other bots are gaming the system. I'm all for abusers being called out for the abuse too.