Mostly the DTCC, which is playing Penny Stock Police by DTC-chilling any stock which sells unregistered shares via the 504 exemption, when it's a legal way of raising shares. Tracking frauds is the job of the SEC, not theirs.
Penson has also done some wrong moves, such as restricting online trading of a stock to 10% of the 20-day ADV which forces illiquid stocks to be traded via phone. That, however, is because in order to clear subpenny stock orders whose size is 10% 20-day ADV, the NSCC imposes outrageous deposit requirements on clearing firms until the trade is settled.
The formula to calculate the NSCC deposit requirement is (($0.01*number_of_shares)-trade_value).
As a example, to clear 3.000.000 shares of a $0.0001 stock (a trade worth $300), clearing firms must deposit $29,700 at the NSCC until the trade is settled at T+3. That requirement is plain nuts.