I believe there should be an option to report spam in the support desk, not only for the RMT tells but for people who abuse Party Finder.

Where/How do I report people who use Party Finder for things unrelated to the game?

I don't know if/where it would fall under, but the ToS:

3.2 Disruption. You may not in any way disrupt or interfere with the Game experience of other players, including the disruption of Square Enix's computers and servers.

3.9 Non-Commercial Spamming/Spimming. You may not use (or abuse) the in-game chat and message services to harass other players.

Are these acts suppose to be reported under harassment?

Making a party finder to say hello, good morning, or typing lyrics to a song is annoying to go through while looking for in game content.