One approach to countering sock-puppetry as a general problem is with increased transparency. Personally, I think lobste.rs' notion of a user tree is a step in the right direction: https://lobste.rs/u
Invite only is awesome, but open forums are important too.
Reddit had a similar problem with truthers taking over discourse at the beginning, luckily it went away. Perhaps because there was a popular parody account mocking them all the time (something like 9_11_was_an_inside_job), and that account retired in time when it's job was done.