How do I stop the monkey madness?

adb shell
ps | grep monkey
kill process_id

[Nitpick] You're confusing monkeyrunner with monkey.

The monkeyrunner tool is not related to the UI/Application Exerciser Monkey, also known as the monkey tool. The monkey tool runs in an adb shell directly on the device or emulator and generates pseudo-random streams of user and system events. In comparison, the monkeyrunner tool controls devices and emulators from a workstation by sending specific commands and events from an API.

[/Nitpick]

On my Android 2.2 device when I start monkey, I see a process started in DDMS by the name "?" (just a question mark). When I killed that process, the monkey madness stopped.

You can kill the monkey process just doing this:

$ adb shell ps | awk '/com\.android\.commands\.monkey/ { system("adb shell kill " $2) }'