Coinmarketcap already does exclude some volume, but they can go a little bit further than that.
It's good that they exlude volume from zero fees exchanges, since people can just increase the volume exponentially.
You can see it when you click on the exchanges tab for each coin.
But when you are seeing another marketcap site and there was a lot of gap between CMC and another marketcap site. i don't talk about coingecko at this moment. there was a marketcap site that only counts the real volume but i forget about the name of that site.