How to erase old or unwanted data

to erase the data for a toon (bank in this case);

open the bank window
click on the Switch Character icon
mouse over the toon you wish to erase the data for

either

click on erase location data to erase just this location

click on erase all data to erase everything for the selected toon/guild

if you wish to just erase data for a specific location (eg the vault for an old guild) you will need to open that specific locations window first.

you do not have to be logged into that particular toon to erase their data. in most cases you want to do this for toons that no longer exist, or vaults you no longer have access to.