Stopping the sending of gil through the mail system, even by level one characters, would be crippling for me. I have two service accounts, and use the second for a group of mules that I use to retrieve items from NPC vendors in different cities. This dramatically reduces my gil expenditure (while leveling all crafts at the same time), by eliminating wasteful teleporting. I have to occasionally send these characters gil so they can buy the items.

Disabling shouts and yells up to a certain level will only force the gill sellers to work a little harder. It won't stop them. The only real solution I can see is to either have the STF (SE Special Task Force) station at least one character in each region at all times, or recruit players to supplement the STF. Increasing the GM staff in order to handle support tickets wouldn't ba a bad idea either...