The /isvalidresponse would allow you to define a valid response as one that occurs after a certain interval of time. For example, the following would disallow participants from leaving the question until 30 seconds had passed:
/isvalidresponse = [openended.example.latency >= 30000]
Or, you could also use the /iscorrectresponse and show show a feedback message if the next button is clicked too early.
/iscorrectresponse = [openended.example.latency >= 30000]
/ response = correct
/ correctmessage = (sometext, 0)
There may be a typo in the code above since I haven't compiled it, but hopefully this gives you the idea. Note that this doesn't disable the next button per se (that's not currently possible with Inquisit), but it should prevent the participant from leaving the question too early.