The prospective guests who don't get back to you after you message them back have either changed their minds, are bad (untimely) communicators, or have decided to book something that is more appropriate to their needs.

The reminders from airbnb re pre-approving the guest (which you wouldn't want to do until you are satisfied that their requirements fit with yours) are computer generated. No human at Airbnb is aware of what has gone on in your message stream with a potential guest, unless you have a major issue with the guest which you have notified airbnb about and they investigate the issue by referring to the message thread.

You can ignore the pre-approval reminders (nor do you need to Decline), which will expire. If the guest gets back to you after that, and you want to pre-approve them, they just have to put in a new booking request.