You can require anyone trying to sign up to complete a reasonably difficult programming problem. It could either be sandboxed on the server, like Facebook's eng

You can require anyone trying to sign up to complete a reasonably difficult programming problem. It could either be sandboxed on the server, like Facebook's engineering puzzles, or require a short output like Project Euler. This would drive away people who can't do FizzBuzz and nontechnical people who want to hire someone to make Facebook for dogs or whatever.