Inquisit's *default* is to erase stimuli (i.e. overwrite them; cf. the /erase attribute) at the end of a trial, which normally is when a response has occurred. This, however, is not what you want here. You want to keep stimuli on screen for a fixed period, even if a response has already occurred. There are a couple of different ways to do this, and I've outlined one in the example, specifically using the trial's /responseinterrupt attribute to keep the stimulus presentation sequence going after response and -- here -- overwrite the stimulus after the desired amount of time.
Hope this clarifies.