You can't stop gilsellers. The only thing you can do is restrict their actions, making it harder for them to the point it's not profitable. Now it comes to the part where you have to carefully choose those in order not to hurt your legit community. One suggestion taken from other games was the options to block incoming tells from certain levels range. Spammers are lv1 chars that get deleted every few mins and remade after, even reporting them is useless because they'll no longer exist. Give people the option to block them directly at Lv1. This would force spammer bots to either give up or level up. Leveling up means time lost for them, and then getting banned after spending time in leveling up, not profitable.
Some say this is very newbie unfriendly. But truth is, the way it is going right now, we have the Busy status, which basically works the same but it punishes us, not being able to be traded, tells from friends, invited to parties or whatever. So far I've never got a tell from a newbie asking stuff, they usually ask on Shout. Also in the hypothetical case I don't want to be whispered or "bothered" I would ignore those newbie tells anyway, so this wouldn't really hurt them, but it would help the rest as we're not obliged to reply.