See e.g. http://hydra.nixos.org/build/4915744.

P.S. existing active build steps of finished builds can be marked as aborted by running:

update buildsteps set busy = 0, status = 4 where (build, stepnr) in (select s.build, s.stepnr from buildsteps s join builds b on s.build = b.id where b.finished = 1 and s.busy = 1);