I like the idea, had folks that should have known better blow a sneak on me.
I don't know about stopping them from taking off, maybe if the fleet is under command a message box could appear (similar to mission scoring choice). This make the guy aware that the fleet is on a specific mission rather than just idling by that enemy base...
How about a CV_MOTD, provided by the CV Commander, similar to squad MOTD, stating specifics...some of the new folks don't understand how things work.