I've never joined an FC myself. I got spammed countless times when I was a newbie but the unsolicited FC invites seem to have tapered off once I hit 70 and completed Stormblood.

There are a few ways I can think of to mitigate FC invites:

1) Clicking Party -> Party Members -> Right click your name -> Edit search info -> typing in some sort of message in the blurb that you're not interested in a joining an FC

2) Spending most of your time in low traffic, high level areas. Most FC recruiters are concentrated in the primary three city states (Ul'dah, Gridania, Limsa Lominsa) and in low level areas where they hope to recruit newbies.

3) Being in and out of duty finder. Cross-world players can't recruit you into their FC because you're on different servers.

4) Spend as little time walking around as a low level job/class as possible in low level areas. I get more unsolicited FC invites when I run around as a lower level job than when I run around as a lvl 70, despite the fact that I obviously lack the sprout icon next to my name.