If a visitor is banned, and the visitor remove all browser information (cookies, history etc.), the visitor is not banned anymore. Please fix it, thanks.
This isn’t a bug – as there are two types of Ban.
1. Ban the user by Cookie/Session, which as you have described if they delete their cookie or go incognito, then yes, they will be able to initiate a chat again.
2. Ban user by IP, which means that they would need to completely refresh their IP from their ISP in order to become unbanned.
We do not default ban by IP, as a visitor may be connecting from a network. Eg: A workplace with hundreds of people connecting from the same IP, so be very careful when banning a specific IP address.