The president cannot impeach them unilaterally, and is explicitly out of his power.

He could, however, potentially send them to a blacksite as a prisoner or conveniently kill them as part of that arrest. They could claim collusion with domestic terror groups, espionage, corruption, etc, as very plausible justification for arrest, and that would probably qualify as official duties, at least how this SCOTUS would classify the same actions if executed by a republican president.