As reamon correctly pointed out, its quite tricky and not advisable to kill the threads.
But i still feel development enviornment is play box and you can play with the new option given in wm 8.
for killing the thread, server property “watt.server.threadKill.enabled” should be set to true from extended setting, i guess default value for the same is true only.
Then click on System threads from the statistics page on IS-Admin and look for the thread you want to kill, be advised that threads related to custom written java/flow service can be only killed but not the system threads. againts the running thread you will see option for killing/canceling it. By clicking on that you can do the same.
PS: killing/cancelling be only available for the theads where threads can be killed, its not available for all running threads on IS.
hope it helps.