Mining pools have begun blocking transactions from or to addresses sanctioned by the US Treasury Department's Office of Foreign Assets Control (OFAC) - they are simply not added to the blocks.
There have been 6 similar cases over the past few weeks. One from ViaBTC, another from the Foundry USA pool, and four more from F2Pool. The most interesting thing is that not only American mining pools are involved in this, but also Chinese ones.
The attached picture shows the hashrate distribution; the top 3 pools can block any transaction.