(Off topic from weapon balancing, but this does balance the economy, which would balance weapon control)
I suggest giving the players a 1 month advance that ZC is being removed. Let them do what they must, let the rage chill, then announce that there will be a conversion of ZC- 5 gralats/zc.
Afterwards, sell a new top-tier gun to reduce the influx of gralats in the system, or hold an auction.
Actually, yea it'd be more efficient to hold an auction as the people who had 10000k zc would still not be nubs again cus they'd have auction items at probs a less steep price.
Cus i don't think that trading is an issue. ZC is.