The best would be not to explain the meaning of the word “constructive” but to demonstrate it.
Saying “sorry, this is not constructive” IS, well, not constructive 
For example, in response to:
This update is awful. The responsible team obviously doesn’t care about their jobs.
Say:
So great we have some immediate feedback here! May you please elaborate a bit, what exactly is awful: we wanna keep our users happy, but your feedback needs some more details before our team can discuss it.
Don’t even mention it in your answer that it is not constructive – by saying so you’ll basically state that the writer is incompetent in giving any feedback in written form; to many people it may sound offending (i.e. you are publicly saying that what they wrote is inconsistent or not useful).