OP here, I just called Pepco and Constellation.
I had to convince Pepco that they were not talking about Energy Choice. The lack of printed material and the "I need your account number" helped the case, I guess. They took down some information and asked for my contact info in case they needed more details or to follow up, etc. They thanked me for calling them and making them aware.
Constellation had no clue what to do, and it sounded like the girl was simply going to ignore it. She kept saying, "ummmm... ok" and "I don't know what you want me to do with this information." I kindly suggested that they put something out to their customers stating that "Constellation people will never come to your door and ask for your account number" or "If someone claims to be from Constellation, ask them for (this thing)".
If nothing else, this proves that Pepco's customer service HAS improved markedly
