You really shouldn't block nestboxes when they are laying. Otherwise you'll inadvertently train them to lay elsewhere, which could be a problem. If no one is laying (and no one in the flock currently has a bright red comb and/or is squatting) you can temporarily block the nests.
Young pullets will spend more time in the nestboxes, when they are approaching point-of-lay, in other words, they are getting close to beginning to lay. You can put fake eggs (I use golfballs) into the nests and leave them there, which will help alert them to the fact they should lay their own eggs there, when the time comes.
The real big poopy nest problems happen when birds spend the night in the nestboxes, since they poop a huge amount during the night. During the day, they are becoming familiar with the nest, as their laying time approaches. I like to leave the nestboxes open for young pullets to investigate, but make the sleeping roost easily accessible, draft-free...and comfortable for perching throughout the long night (this means using a 2x4 with the wide side facing up).