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 ReadyApi and start it all over again.
This happened to me a few times and it's pretty annoying.
Here you can review submitted feature requests and vote up the ones you like! If you can't find the feature you want - go ahead and suggest your own idea. Ideas with the highest rating can be implemented in the product.