Block off the hole that is most difficult to mount a one-way door, and then mount the door to the other hole.
said by TheSMJ:Besides, if I put something up that allows the squirrels to leave but not get back in, it would be as though I just blocked off the holes while they were out feeding during the day. They'd come back all peeved that they can't get back in, and start wrecking my roof looking for another way. The only way to prevent that would be by removing them from the area.
You are over thinking this. The squirrels will find another home when they cannot get back into your house. Yes, they will be peeved but they cannot destroy your house trying to get inside. They have to find shelter, food, water, and a new home. They are not going to shred your roof. I know from experience.
A cage is great if you want to deal with catching, transporting, releasing, and then hoping you caught them all before you close up the attic holes. Accidentally miss one and close up the holes too early? Your house will stink from the rotting squirrel inside.