The way to stop unclosed quotes is to prep your customer on the survey.
You need to be asking more questions about why they are getting the quote. WHEN ARE THEY CHANGING Price, quality etc etc
You can never ask too many questions
In your case, just be direct "did you contract with anyone yet"?
NO, "when will decision be made"?
YES " would it be ok to stay in touch and see how things are going"?