Short-term, intense focus on this issue would make a huge difference. Spend one week and two weekends cracking down, every day. Get members of the graphics team, the combat team, some help desk monkeys, and a handful of interns to be visibly in game EVERY DAY during that time. Patrol the known botting routes (Grahtwood, Rivenspire, Reapers, Stonefalls) and zap bots. Over and over and over. Hell, maybe address the worst offenses in zone during that time too. That's all it would take to get a reputation as a company that has started to take it seriously.

There would still be bots, but fewer. Gold selling companies would see ESO as more trouble than it's worth. If they are currently able to keep each account for a week (I'm projecting) before Zeni responds to reports and bans, it's still profitable. If they are burning through purchased licenses every day, especially multiple times per day, it's a losing investment and they'll stop.