Oh, I see.
I have the base game and I got a key for a friend (I was willing to play with 4 people, but apparently they didn't get in, in time...)
From what I can understand, the free key he got (Multiplayer base game?), has steam cards. I imagine the system was flooded due to people willing to get free cards and selling them.
Maybe remove card drops from the free/multiplayer game? That will probably make it uninteresting for people to spam the system.
I hope this helps! And I hope I can get some friends to play a few games with me in near future and get them hooked up