Im at Wldc and , yes, you can trade the SPY, QQQ and DIA. And you can trade them through ECN's (ISLD, ARAC, INCA and REDI book) - same rates, no ECN charges.
However, those stocks are on what is called a RESTRICTED LIST. This list is an on-going list that has abut 15 - 20 stocks that many of the new/"negative capital balance" traders can not trade. This means, if you are without capital or little capital, you will not be able to trade them. If you have capital, then you can do whatever you want.
You can call them directly to ask them about htis policy but it is something that they are very serious about. The reason for this list is that these stocks (which include SPW, NOC, LLL, MER, GS, IBM... among others) are known for being very difficult to trade and the firm has lost lots of money net net from the traders trading them. In retrospect, it is a good policy. SPY is probably my worst stock ever.