When I first heard "Stop the Secondary," a few things came to mind. Would this take its concept name literally and STOP secondary moves/users? If so, does that mean that all it does is switch into secondaries, which would probably mean Taunt or a custom ability version of it, or stop it for the whole team, meaning a semi-permanent version of Disable?
Maybe you want something akin to petrie911's concept of "Turning the Tables," something that takes advantage of secondaries? That made me think. The first thought was a custom ability that upped stats the less hp you have (entry hazards), and upped it even more when afflicted with a status (uh...paralysis, burn, poison?)
Seems to me that, the way this discussion is going and having Dark as the primary type (correct me if I'm wrong, I didn't read the whole thing), people would rather switch into a secondary move, kill the user, then switch out. That, to me, sounds like Natural Cure, Arena Trap or the above mentioned custom ability is what's needed. If all you're going to do is get in, kill secondary user, then get out, it seems to me that you might as well do something else "Stop the Secondary"-esque while you're in there and absorb Toxic Spikes.
Bash me if I'm wrong, but POISON sounds pretty viable to me.