I definitely noticed a huge decline in gil selling chat spam when they introduced the report function. It used to be teleporting to any city would cause your chat log to instantly be filled with spam, and even if you weren't in a city they would constantly send you /tell messages. After the report function was introduced, the spam started slowing down until it became virtually non-existent. These days I only see the occasional shout in Limsa (never the other cities) and I haven't received any /tells since.

Unfortunately, it seems that the only purpose of the report button is specifically to combat the chat spam, not to actually put a stop to RMT. If you report someone for chat spam, a GM can easily check the logs and see right away to see that they're breaking TOS. For the gil farmers, while it may be incredibly obvious to us when we see a group of them what they're doing, the GMs have to actually do an investigation and prove that they're doing something wrong, and not just a group of legit accounts with names like "Xcusidonnd Applef".