I think the best argument against climate change denial is the free market. Anybody can spew nonsense arguments on social media, or even on the floor of congressional chambers. But when real money is on the line, do companies deny climate science? I propose the creation of a climate change denial fund. A mutual fund/ETF that invests in companies and perhaps some commodities that would benefit from climate change being a hoax. Then when confronted with a denier, we can ask them to put their money where their mouth is and invest in this fund.
You might retort that there are already pro-climate funds. This is true, and theoretically you could say that shorting those would be equivalent. I think a purpose built denial fund would be superior though for teasing out true positions from green-washing.
The best argument against this is that asking a politician to invest in such a fund creates perverse incentives. But I think it would still be a good argument to use against non-politician deniers. Also just having the price of the fund to point to is well worth creating it.