Depending on the circumstances, you may tell the participants that the session is finished. In other cases, it might be appropriate to ask the people in attendance whether it has finished. The more unstructured the session, the more likely that you could use the latter approach.
Don't be afraid to say that it is finished if people appear to not be willing to let go. You can do this by stating that anyone who wants to continue discussing the topic of the session is welcome to stick around. That lets other people know that they can go.