If you get a bunch of messages (some you simply don't want to answer / have no interest), you will have "left the PM." Could be interpreted as rude, could even stain future interactions / business. A small detail, but I'd say remove the fact that its closed, so people don't interpret it, in some negative light. If people don't get a response from you, they'll have got the message anyway, without you having to keep the DM alive. Then again, maybe its just me, that thinks it has a f*** you vibe about it, by having "left the PM." Just makes things a little more nicey nicey, taking that statement out of the equation (maybe)