Civil and constructive? I'd say ignore any negative comments and block them so that you don't have to deal with them ever again.
When someone makes up their mind that you didn't win fair and square, anything you say to them only stokes the fire a little more. If you really must respond, tell them to report you and let the chess.com staff settle the matter if they really feel strong enough about it.
Dear fellow chess players,
I am not a great chess player. My USCF rating from long-ago tournament play is 1806, and my Live Chess speed rating hovers in the mid-1800s. I play to relieve stress, guage my mental accuity, and just have fun.
Having played on other onlne servers, I am delighted at the generally high level of civility, if not downright friendliness of people I play. This is compared to the often rude conduct on servers like ICC or Yahoo chess.
Occasionally, I do get people accusing me of using computers to help me find moves (which I don't even know how to do). I respond calmly, explaining that all the flaws are mine, but often am not believed. Maybe these are just the rare sore losers, or maybe something leads them to think as they do.
Any thoughts on how to civily and constructively deal with such situations?