Add that person to your friendslist?

I get this message from time to time, if someone PMs me and they have an IGN similar to someone in my friendslist (there's a similar message for people with an IGN similar to someone in your guild). Annoying cuz sometimes it actually is someone I know, just on another char. I don't know why RO does this, or how it decides if two IGNs are "similar" but that's how it appears to work anyways.
A quick Google turns up
this, and there are people there who say what I kind of assumed when I first saw this message - that it's some kind of anti-scam thing, although it mainly just serves to annoy

Still, no idea if it's turn-offable, although you might try using that one dialog where it lets you select which messages are displayed - doubt that will help any, but it's worth a try perhaps.