Where to discuss the show after IMDb shuts down the boards? ...

I hate reddit with a passion but sadly it may be the best option. Granted I felt that way about IMDB a lot lately as well but the Kiljoys board was never to terrible.

There's a Facebook page: https://www.facebook.com/groups/KilljoysGeeks/. It's a closed group, but we're a fun group and we discuss Killjoys and their actors.

There is also this site.
http://forums.previously.tv/forum/1653-killjoys/

Here's another option:

TheMovieDB.org (TMDb) has forums for all
shows, movies, and people in its database.

Here's their Killjoys page:
  https://www.themoviedb.org/tv/62413-killjoys
and here's their board for this show:
  https://www.themoviedb.org/tv/62413-killjoys/discuss
    (currently empty, waiting for posts)

Recent posts from their other boards can be seen here:
  https://www.themoviedb.org/discuss

Support discussions are here:
  https://www.themoviedb.org/talk