Dude the idiocy of people knows no bounds.
It is OKAY to act in character and have fun. Sometimes things get out of hand - I will admit I’ve done my share of cringe-worthy things in the name of “being in character”. I felt horrible afterwards and apologized profusely to those I may have embarrassed or harmed.
Just know that there are people like this in every fandom. You’ll hear people point out “Oh Hetalians” ”Oh Homestucks” ”Oh Narutos” etc etc. But they’re in every fandom.
The way to stop it is to be firm. If you SEE SOMETHING HAPPENING, you say something. Call that shit out. Don’t just be like “Oh that poor Russia, the way that Belarus is hanging off of her/him…” NO. You call them out. Be firm about it. Say straight up, “You need to fucking stop.” If they do not cease then you get con security.
People use conventions & cosplay as a way to break social norms. Of course you can’t do these things to people you’ve just met out of cons - that doesn’t mean it’s suddenly okay in con.
Please everybody. If you are uncomfortable with someone’s behaviour, if you see someone uncomfortable by someone else’s behaviour, or if you yourself notice that somebody is uncomfortable with your behaviour, you call that shit out. If someone says something racist, offensive, and otherwise something that might get them looks outside of con, it is NOT suddenly okay to say/do them in costume in the name of “in character.”