They recently added some features to fight this in 7.0:

- Blacklisting is now account-wide (blacklists all their other characters, not sure if it persists after they delete the character or not, I suppose that could be a potential oversight?).

- You can now hide your lodestone information so only certain details can be seen, or so only certain people can see details (such as friends only). Thus allowing you to transfer.

- I believe external websites that track information (ie. damage) about you have opted to respect this privacy feature, but in case they haven't, you should make sure they hide your data as well by specifically registering and configuring it.

- There are features to expel people from FC houses.

- If you contact a GM, they can apply an in-game restraining order to them which, if breached, can result in a more severe penalty. If it is the same overall account doing it, then it should be possible for the GM to still track their account across characters.