You've done it correctly, although the tithe alert setting is not global -- are you turning the setting off for each character?
You can hide the alert panels permanently by editing the XP bar file. In eq2ui_mainhud_experience.xml, use Notepad to search for two lines very near the bottom of the file. Search for:
On each of these two lines, near the end, you'll see
which you'll want to change to
This will stop the solid and flashing tithe alert panels from appearing. Please post again if it's still giving you trouble.