Download this addon and install it in your "/interface/addons" folder:
Configure it with "/st".
Gold seller spam is annoying. SpamThrottle is an addon which will remove unwanted messages from your chat window. It looks for keywords in chat messages and if one is found, that message won't be displayed. Tired of seeing ads for "buychinesegold.biz"? Add that as a keyword and you'll never see them again! It's great for blocking gold spammers, but you can also use it to block anything else you might want to avoid hearing about, like "thunderfury" or "alexensual".
Unfortunately, gold spammers are nothing if not persistent. Once they realized that people were blocking their chat spam, they started spamming their ads through channel invites, something that SpamThrottle doesn't handle.
Luckily, a clever Redditor named atterowins came up with a solution: a snippet of Lua code that blocks all channel invites. You can append this code to any addon of your choice (i.e., the aforementioned SpamThrottle) by opening the .lua file and pasting it in, or you can simply download an already edited version of SpamThrottle (uploaded by Alexensual), linked above in the "TL;DR" section.
Please note that I am in no way claiming any kind of credit for any of this. I didn't write the addon, come up with channel invite blocking code, or combine the two and make them available for download. I just made this page as a simple way of passing on the information to other players who might benefit from it. Not everyone has a chat addon that lets them copy a url, so I figured it would be more convenient for them to type "fuckgoldspam.neocities.org" into their browser than a long url from Reddit or the Elysium Project forums. If you find this solution to gold seller spam useful, please thank the original authors for their contribution to our community!