When any of the selected triggers occur the campaign will be assigned.
Yes, you visit the user's profile in the AdminCP, then scroll down and look at the bottom left hand corner of their profile. You'll see which campaigns the member is a part of, and be able to add the member to new campaigns, cancel existing campaigns, or restart the campaign.
If you have a user on "Campaign A", they cannot be added to "Campaign A" again. You can cancel the campaign if you wish to stop it, or restart the campaign if you wish for the user to start over for some reason. In practice, you would typically only want to send a series of emails to a user once however, so this shouldn't be a major limiting factor in real world use.
If you disable the campaign it will be stopped for all users (however their progress will be remembered in case you wish to reenable the campaign later).