Make your own group
Plunder ex-leader
Profit
???

I guess he could join the group that is most frequently plundered by his former leader and diligently provide support such that it makes his leader fail at plundering more. That's kinda weak thou and won't have full coverage...
Maybe if the ex-leader is a heavy plunderer, maybe he could just try taking his coins after he's down his round of profit. Then again I don't know his power or anything, or if doing any sort of personal 'revenge' is in the question.
To be honest I've never really had this issue, seems lame though. He could be able to befriend some top players who aren't a part of that group and just call out a hit mafia style.