I've finally just turned off Tells altogether in my General chat log. I get about 1 gil spam tell every 2 minutes.

It would be nice to have a feature to turn tells off from players below a certain level, along with white lists where you could exempt friends, etc. I don't think gil sellers would like having accounts banned with fully leveled characters. This has been suggested / requested of SE for years now but clearly they are not interested in fixing this. I've heard before that gil selling spam is really only an issue on the NA / Euro servers. That may be why our Japanese developers don't seem to give a s**t about this issue.