Wipe their faces thoroughly after they finish their milk, then give them something to distract them from the navel sucking. Playing with fresh straw is a good one, and as soon as they’re old enough, a bit of cake to mouth. I used to use Carrs Sure Start, which is safe from quite a young age, and put a bit in their mouths as they came off the milk. And a bit sprinkled in troughs, buckets and so on, for them to find and tussle over.
If you see them navel sucking, distract them.
If you haven’t time to distract them and they’re not interested in the cake yet, then yes, individual pens for an hour or so after milk, just to break the habit and association. (But always wipe their faces clean or the milky smell will get them going again when you put them back together again.)