First, they want to roost as high as they can get, so if you make your roosts higher than the nest boxes, those will become a more appealing place to sleep. Second, if you can block off the nests at night, that will discourage them. I actually just installed my nest box unit in my new coop today and I'm going to use feed sacks lightly tacked to the top to block off the nests at night. None of the birds currently in that coop is laying yet but one is looking like she is almost ready so I want to leave her access during the day so she can check them out, but block them at night so they can't sleep in them. I figured feed bags that could hang in front during the night but be flipped up and over to open them up during the day, would be the way to go.