If bots start using a whisper/tell system, I'd recommend against blocking them for at least a day or two after reporting them. Some Bots are capable of seeing invisible flags/system messages normal users cannot see, and may delete their character the moment they receive such a message stating you've blocked them/etc. They may then instantly create a new character and start doing it all over again.
We had this issue at Aeria Games, especially for games made by X-Legend such as Grand Fantasia. GMs were yelling at people to not block bots, as they found that this is what they were doing. In PSO2, there are also presets for chat. You can change to a preset that doesn't include the chat type the bots are using to soft-block them in a sense.