If you block them in with babies inside they will eat their way back
in. Blocking a gable vent can lead to mold in the attic. Moth balls in a
sock with string attatched-to retrive later on if needed, and a bucket
of Amonia might help, if a after a few days you dont smell amonia add
bleach, it makes a poison gas that will drive them out. I had squirrels,
bats, flying squirrels , in the attic for awhile.