Thank them for their response and ask for more details. If they are serious and they know what they are talking about, they will respond with more details. Otherwise, they would just stop responding.

Getting feedback you disagree with is great. That's the most useful feedback as it tells you something you don't already know.

Try to really understand why they believe what they believe. Chances are your viewpoint isn't perfect, so by getting to understand someone else's you get to readjust yours taking into account their perspective.

Also, often it seems like you're in disagreement, but really you're talking about (slightly) different things. By diving into the other person's reasoning you can quickly tease that out as well.

Check people's credentials before asking for feedback.

Don't take advice from people without credentials and/or without current or previous skin in the game en.wikipedia.org/wiki/Skin_in…, preferably in your industry or niche.

E.g. don't take advice from people who haven't done or aren't doing the same thing in the environment you're doing it. This applies not just to business.

I would listen to them and thank them for their feedback. There is no reason to get defensive, and there is nothing to be gained by wasting the emotional energy trying to argue with them. They may be doing you a favor by letting you know that they aren't the right market fit for your product.

I would still log their feedback in a GitHub issue and archive it. That way, it's searchable if you ever want to revisit their feedback. You might eventually see a pattern and find it to later be of value if you need to pivot.