Perhaps "combat" it is the wrong word but as the title suggests, I am in a bind. My group of several adventurers (typically the same 4~5 every session) are great when they work without a certain player (for privacy we'll name him Steve). Steve, at first, played really nice with the first 2 players - which would increase to 3 not long after. The "original 3~4" players seemed to work with this beautiful cohesion that I'd never before seen in other campaigns that I GM'd, and I was thrilled as they would walk away voicing begrudgingly that they wanted more after a session was over.
That's the best thing you can hear as a GM.
However, after about the 9th session or so (my group meets every weekend just about) I've had the opportunity to invite a few more players to even out the scheduling conflicts so that it turns into a fairly consistent "round-robin" of PCs. There are the "iconic 3" still there - Steve and 2 of his 3 compatriots. But at some point, Steve just went off the rails.
Story-wise I couldn't make sense of his character's actions. He would consistently split the group by walking off on his own, seeking his own loot, profit, glory, or what-have-you. He would steal the spotlight in unflattering ways, and consistently (even to this most recent session) get the party into conflicts. I typically, for the sake of the other players (who total about 6 including Steve) try to take the edge off of what natural consequences would occur "no thanks" to Steve's... well, blatantly stupid decisions.
What was most disheartening was when I saw around the 11th~12th session, when Steve couldn't make it due to family obligations, that the group had the most fun they had in a while. No longer were the party members following after the trail of destruction he wrought, no longer were several of them timid (my wife, included) and they got along swimmingly. They got a chance to breathe and do what they wanted to do.
I feel that since I treat everyone equally when it comes to actions and reactions, no one does anything because Steve can simply become overbearing and the campaign quickly runs away with him.
Any suggestions on how I can improve this situation? (I can clarify later if needed.)