If they have to use your stutter to belittle you in an argument, especially if they are purposely starting arguments just to do this, they are low brain wavelength. The best response in my experience is to very calmly point out what they are doing: they are using your stutter to belittle you, possibly in front of others, because they are self conscious and weak, they have nothing going on for themselves and they feel like they have to belittle others in order to feel big. If you calmly point this out, especially in front of their usual audience of enablers, anything they continue to say against you will only prove that point. It may take practice but if you point out the fallacy in their own self they will have to reevaluate themselves