These are called decorations. They are on by default but I guess we could delete them.

I changed the numbers around so that now users will get a green star after 1000 posts, magenta after 2000, blue for 3000, red for 5000, silver for 10,000, and gold for 20,000. Someone with 4000 posts will have a blue (3000) plus a green (1000).

It's fairly useless but also harmless -- so I can leave it on or delete them. Doesn't matter to me, as I never look at them unless someone asks.

BTW: I see 2470 posts for jakescia. After editing the decorations, his account shows one magenta (purplish) star. Before, it was one silver (2000) plus four magenta (100 each) plus two greens (one for 50, another for 0-25).