Very simple way to stop all popups is to install uBlock origin extension, that works both in Firefox and Chromium based browsers.
With this extension you won't see 99% of ads on whole interenet, not just CoinGecko, but you can also use scripts that block ads or disable javascripts (this can break websites).
I don't see any advertisement on CoinGecko but I guess they are making profit from this advertisement, and with creating account you can collect candies and get some discounts, so it's not all bad.