If other items come up, table them and add them to the end of the agenda to be discussed in any allotted time remaining at the end of the current meeting, or schedule another meeting, or delegate the item. Establish a start and a stop time. Start on time, even if someone isn't there. Don't penalize punctual people and reward tardy people. Control the meeting so that the business is concluded by the stop time.
Some people monopolize meetings.
It is your responsibility to ensure that everyone gets to speak, and that the long-winded ones get to make their point, but not over and over. You must also ensure that the meeting stays focused on the agenda. It is easy to get off track and reach the end of the meeting time without having accomplished its purpose. If you really want a short meeting, don't provide chairs. Have people meet in your office.