Most games that implement bans do so more because the chatspam becomes an annoyance to basically everyone. WoW, for example, allows it so long as you don't advertise in /1 and the like. (At least, that was the policy as of the last time I played.)

I don't see any specific casino policy for FF14, but it's worth keeping in mind that spamming chat with advertisements might get you in trouble anyway.

EDIT: This is, of course, assuming that it's legitimate. If you're doing it as a scam (and I'm sure a large enough number of scam casinos would lead to a ban on the practice as well), then you're obviously breaking the rules.