Quoting Squid Kid:" Like people telling you you shouldn't try again after a miscarriage or it's too soon or that"You know ... [snip!] ... are people who act like I should only have one or that a 2-3 year age gap is too soon or we're trying too soon after the loss."
I'd tell them it's really none of their business.
I haven't been through what you have, but I know after having Teagan (being a preemie and all) that I got a TON of comments about not having more, or waiting, or things like that. We didn't tell anyone we were trying and made it clear it was no ones business.
We got a few bad comments once I was pregnant, but not much.
I'm sorry hun, it's rude when they do that because it really isn't their business. It's easy to look from outside and say "you're done, don't have anymore." I think a lot of times it comes from a caring place because they don't want to see you hurt, but it isn't their business. People from the outside only see risk, or pain, they don't really "see" the desire to complete a family, or the desire to have another child.
I think the only way to deal with it is to make it clear that it isn't their business and isn't up for conversation or debate.