Current Javatest Agents live forever after launching. The only way to stop one is to kill it manually.
For automated test runs our internal groups use different wrappers which track agent's activity and decide whether it's time to kill it or not...

Introducing a mechanism for stopping the agent would help a lot.
The simplest way is to stop the agent after the pointed amount of unsuccessful attempts to connect to Javatest master (works for active agent only).
Another idea is to use the "inactivity period".
A more complexed one is to await the special command from Javatest master (or introducing a control channel for remote termination).