As long as contact is maintained, they're vulnerable to conventional attack. So if he's got one in a headlock, it'll do damage. However, they have to be touching bare skin (blah blah comic book science electrical currents in human skin blah) in order to accomplish this, so it wouldn't work if he was fighting in his suit.

The lingering effects, if any, of possession, are sort of up to player discretion. If they were shocked to expel the spirit they'd expect damage on the scale of what I linked above. If they fought it off themselves they'd probably be exhausted. If you want any other effects of the mental or physical variety, feel free to pick anything that seems to fit for both your character and the scenario (ie: nightmares/etc.) I mentioned upthread that we're leaving whether or not people are lucid while possessed up to the individual player.