Once that has been unlocked, apply the permission node 'economytax.tax.avoid'.
To apply it to a certain player use:
/setuperm <player> economy.tax.avoid
To apply it to a group use:
/setgperm <group> economy.tax.avoid
NOTE: You will have to be in a custom group (made with /addgroup) in order to use the commands '/setuperm' and '/setgperm'.