I searched for this and found 1 post and it said do /ignore but you can also do /invite ( name of horde or ally player).
If the person is offline your looking for it will say : cannot find player * name *
If that players online it will say : Target is unfriedly ( means there online)
so its just a way to check if the player you hate alot is online or if you want to hunt someone down and gank them, if the person normaly farms in a certain place check there for them and happy ganking