I've had that happen before - it's kind of irritating (especially when it's blocking a pre-approved reservation)!
It's held for 24-hours to give the guest time to provide ID or other verification - if they don't follow through, it will just disappear from your calendar. If they pass verification, their original message will come through with the full request.
Unless you urgently need to unblock the nights (like they're this week), you're probably best off waiting for it to clear on it's own. You could try working with Airbnb to force it's removal sooner, but that might be a lot of legwork or hold-time for an issue that will resolve itself within a day.