You can't ban someone without solid proof that they're an RMT, even if there hording gil and doing leves 24/7 theres nothing in the rules against this...you have to wait untill they break a rule (Sell Gil etc.) I'm sure SE has a list it keeps an eye on.

SE will clamp down on this in 2.0 i'm sure, in FFXI they came down heavy on RMT and bots in 2006-08