In a recent video project I came across some "popping" which I believe was in part due to the person being too close to the mic.
Without verbally prompting them, what would help remind them to keep the correct distance? Any tips, devices etc.?
Something that helps in recording studios is to get people to sing over the mic rather than directly into it. You'll notice that they generally hang a condensor mic from the roof and position it at eye level, higher than the mouth. This gets rid of all sorts of pops and plosives, as the wind generated from speaking doesn't crash into the mic.
For a speaking engagement, you can simply set up two mics – one that is a little closer and higher, which you turn right down, and position the real mic underneath it. This helps for people who naturally want to go right up to the mic and talk directly into it – which never sounds as nice as talking over it and turning the mic up a little more. I'd also look at getting a foam 'sock' for the mic, and using a high quality dynamic mic, for example an SM-57.
Hope this has helped.
Sometimes a pop filter is not enough.
If you have an audio engineer on site monitoring the signal while the singers are recording this live or in a studio have the engineer work out a hand signal that can be interpreted as "back off from the mic".
Perhaps a reverse high five or a simple hand retracting would work. The key here is to have both the audio engineer and the talent on the same page to whatever gesture is chosen.
Think about how professional baseball players use hand signals all the time to communicate from coaches to players and players to players e.g. catcher and pitcher. When you think about the complicated signals these guys use this seems like a walk in the park.
Is the audio level clipping or is it actually a pop from breath or spit or something? If the audio level is simply clipping, then a gain adjustment may be all that is needed. If it is actually a problem with breathing on the mic or spit hitting the diaphragm, then you either have to tell them or use some kind of barrier. I don't know any other options.