Actually, I think that because their minds are still present, they could be controlled through Legilimency or an Imperius curse. I mean, other kinds of things, like Basilisks and Lupin's "waddiwassi" spell, can affect them (Although that was technically on a Poltergeist, I think the principle holds true). Based on canonical evidence, it's actually pretty probable that Voldemort could control them. Not just through Legilimency, either. He might use a Cruciatus curse, or other means of physically controlling someone. It probably wouldn't be as effective on a spirit-based being as a human, but it would work.
Originally posted by Hayden:
OOh, also, Voldemort, if he had some ghosts under his command, would have had to have been sure that they would follow his command, because he couldn't control them completely. Anything Legilimency-related would have been impossible...or would it really? As ghosts can think for themselves? They aren't solid at all, so that's not like controlling someone alive.