You make a great point. I think that there needs to be multiple points in the conversation where the user can choose to exit.
Of course there are a set of key reponses that are needed to complete the desired action (i.e. signing up for an account) and perhaps this is where some form of visual marker would be useful to the user.
After that I think the interaction would be similar to the flow of a natural conversation. There are peaks of heavy interactions, and lulls where there is the opportunity to end the conversation. Once the required responses have been gathered it’s the user’s choice whether they wish to continue the conversation or cut things off.