Its all about filters, in an automated system you can add as many conditions as needed in order to polish it to your needs. if guilds are a problem then add a condition that only one report per guild is valid. you could do so for friendlists or parties. I still believe such a system would prove to be efective.
One other alternative is that reports dont inmediately give Ban, but trigger a Bot check system that is able to ban bots, if you get anought reports in a town or field then this system would start recording you behavior (Chat log) or time spent in a especific map area. chat spammers could be banned if the system determines they send simmilar or equal mesages, or their messagues include banned site pages or money transactions.
As a last resort and more user friendly instead of instant ban by system i think a priority list by system filtering should be done regarthless with light sanctions like the ones abp sugested for chat, and the last chioce is made by an GM.
The problem i see without permanent sanctions is that any bot user can simply solve chat supresion by running more characters in town and still making more or less the same spam, of course it does help somehow but i think is not enought