I with you! I was in the same situation and although my only is 13 now, I still sometimes feel, that when the conversation turns to children and pg, I can't say, 'I can't have anymore', for fear of people then getting all on me!
There were times, when people who didn't know my situation said things like, 'oh go on! You've got to have another one!' but tbh, I just made a comment like, 'no thanks, one is quite enough for me' or no, you're OK, I'll leave it up to you' then quickly changed the subject!
If you can let the people that know, understand that you are totally fine with it, then they will respect that and probably not make too big a point of discussing subsquent babies with you, but equally you have to stop worrying about them feeling they can't discuss it in front of you, because they are only too aware of your situation.
When it comes to strangers, its up to you. Lie outrageously and say you can't stand kids really! That'll stop the conversation dead!
Your dd is still at an age, where people expect you to have another, but it won't be for long. Once she hits 5'ish, people will ask less and less, because they are more likely to consider you don't/can't have anymore, iyswim!
Jeez, I'm rambling! There's no easy answer tbh! I've been there, seen it, done it though, so you know you are not alone!