One thing I like to do online is obscure references to old movies. If someone says or does something in a forum that reminds me of a character from a movie I will say: "Hello <character's name>" If the name is recognized people will say so because it is a fun thing. Or they will respond with quotes from that character. Other people will join in on this too. An example of this is calling someone Raymond who estimates the exact number of things in a picture. This is a reference to the character Raymond from the movie Rain Man. This character was autistic and could could numbers of things instantly accurately.

However there is a chance the name is so obscure that it just sounds weird. Nobody gets it. Maybe they recognize it is a reference maybe not. This goes unnoticed and unheeded.

There is an even more remote chance that this is in fact the person's actual IRL name. However, if they are on a semi anonymous platform then they cannot confront the person calling them by that name because that would out them. Also, if they PM me about it they are outed again. So if this occurs this person will now wonder how the fuck someone knows their real name. Silent torture and wondering about it for a couple of days.