It's very annoying to get random FC invites and I think you've gotten what you deserved. Atleast if you make adds in chat, people will be able to send you a tell if they are interested.

Your FC wouldnt happen to be "The Dark Mortals" by any chance, because then I truely understand that you got a ban warning, because that is one overly spammy FC. Infact it's the only FC I can recall getting a random invite from on several several occasions. Me and some friends more or less have this game "Guess who just sent me an FC invite", and damn me in all nine worlds if the answer isnt "The Dark Mortals".