It is not your name. No player owns their character name. Other players are free to use the same name as long as they have a different home world.

There's no way for us to remove a Maker's Signature from crafted items that display one but it is possible to cause the name displayed to change on items that can be stacked by adding items with one name to a stack already in your inventory with a different name.

We're not certain what "Obtaining Signature" represents anymore. It used to represent items crafted by a player that had a home world different world from the one the item is being sold on but that's no longer seems to be the case. Our best guess is that it represents items crafted by a character that has since been deleted but we can't be positive. Only SE knows.

Honestly, just ignore that other player. I doubt anyone else is paying attention to the retainer names. There are a lot of really dumb retainer names in use. By letting it bother you, you're giving that other player what they want.