Internet is global, there are no borders.
If some Chinese wish to express their opinion on Brexit, or some Russians on U.S. midterms, good for them I say. It's their right. Deal with it. What's the best way to fight bad, false information? Maybe with good, truthful information, idk... Centrally not with censorship.
They arn't people expressing their own thoughts, they are paid trolls paid for by the Russian state, posing as UK citizens. Pretty easy to catch them, just post a link in comments section of whatever EU/Brexit article to a server you have access to, some will click it and you got their IP. Im sure in a few years the Russian will stop being so sloppy and use a VPN, but even then it'll show a datacentre owned IP and not an ISP.
Yekaterinburg and St Petersburg are favorite cities for the hot topic on Brexit. It's about 90% Chinese and Russian traffic.