For now there is no possible way to stop a groovy script from executing. If you create an infinite loop for example the application will crash in no time giving you no other option but to stop ReadyA...
I would also like to see a stop button implemented. I have a test case that checks logs after all the requests have been made. This groovy script can take up to 20 minutes to verify the logs for all the requests made. It would be nice to be able to stop it if we have issues.