I don't know if this is what you're referring to, but some members are on Contract. They cannot be fired or reassigned. Most can be manually checked off Contract status.
It should show a Lock, though, not a crossed box.
For instance, this guy.

His checkbox is crossed. He does not have a lock symbol, and he is not in the combat unit. I can't select him, and thus can't reassign or fire him, and I also can't use the "Direct Contract" option. He is also not in sickbay or in the brig.