Processes spawned by a job are/may not be stopped or killed by Hudson when manually stopping the execution of the job.
However Hudson already contains such functionality, the so called ProcessTreeKiller.
To enable it you must set a Java property named "hudson.util.ProcessTreeKiller" to the value "true".
This can be done as a parameter to the "java" binary when starting Hudson:
There may be other ways to permanently enable the ProcessTreeKiller.
The ProcessTreeKiller itself takes advantage of the fact that by default a new process gets a copy of the environment variables of its spawning/creating process:
It sets a specific environment variable in the process executing the build job. Later, when the user requests to stop the build job's process it gets a list of all processes running on the computer and their environment variables, and looks for the environment variable that it initially set for the build job's process.
Every job with that environment variable in its environment is then being terminated.