At a bare minimum, actually interrupting the network connection will end their mouse and keyboard control. If you have only a wired network connection, pulling out the cable is guaranteed to end the session; with wifi, if your computer has a physical switch, turning it off will end the session. Likewise, shutting off the computer will terminate the session; QuickSupport doesn't support unattended connections, it seems, so that would be sufficient.
On the other end of things, the TeamViewer manual says that the remote computer (i.e. the one being controlled, not the one doing the controlling) has a button on the TeamViewer panel to end all active remote control sessions; if your computer is being controlled, pressing that button would terminate the control.