Do this with the discussant as well. People will run over and don't sweat it too much, but at the same time, try to get people to stick to their time limits. Essentially, you want to try your best to keep people to their time limits, but do not get angry or bent out of shape when they don't. Trying to get people to finish within their allotted time reflects well on you. Getting nasty at someone for running over their allotted time reflects badly on you.
Chairs usually manage the questions from the audience as well.
Please keep your questions brief, if possible, so we can maximize the number of people who have a chance to ask questions. If it's a small group, you can also ask people to identify themselves. Again, try to manage things well, but don't worry if people don't comply. Ending the session . Sometimes sessions just fizzle out.