Hi faul,
I think a witty response to a seemingly clever argument is to say that you are giving the carbon freely with the oxygen you are returning, and if they wish not to accept the gift, they are welcome to dispose of it.
Alternatively, you state the obvious. The carbon we emit when we cycle has a different origin from the stuff cars use. Cyclists' carbon comes from food - ie living systems - which means cyclists and all creatures are part of a system that is in balance with itself. And so cyclists do not add fresh carbon to the contemporary atmosphere unless they are losing weight: you consume what you emit. Whereas vehicles consume hydrocarbons from previous epochs, emit carbon dioxide that was previously stored in oleaginous form underground and thus add to the amount of CO2 in the system.
The sensible argument for a bike tax is that cyclists should contribute payments for the roads whose use they enjoy.
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