From /r/bitcoinmarkets
"Today there have been numerous reports through social media and the Whale Club team speak that U.S. customers on OKCoin have had their accounts restricted from participating in the futures markets.
While OKCoin did announce earlier that they were begging to restrict access of U.S. customers due to regulations, there were still loop holes to get Bitcoin into a futures trading account: A) Not generate a new deposit address. B) Download the mobile app and send coins to that address, which you could then use on the futures markets.
Today, OKcoin has taken one more step in preventing access by locking out the futures market completely to U.S. based traders. See the image below to see the notification that you are prompt with.
http://prntscr.com/96pxrb[1]
Bitmex has restrictions in place for U.S. customers to restrict them access to high leveraged Bitcoin trading, but there are still ways to get around these, as many of you well know. As regulations continue to develop in the U.S. and other countries abroad, it seems that people wishing to trade at higher leverages will be forced to find new brokers to participate in that style of trading.
1broker currently offers BTC /USD trading with up to 5x leverage
The market is currently in an experimental state, while we gather data which will be used to develop an actual futures market for Bitcoin in the future.
As a disclaimer, I do work directly for 1broker as their community manager, but I noticed a lot of people wondering where to trade now that a clamp down is happening across all the other major exchanges. There are still choices out there!"