While I don't know Redis syntax (mogodb user here), so I can't give you specific commands, I can assure you that most of what you want is possible to do in the database (I don't know about groups here).
The database structure is explained here, and as you can see user objects contains the website, postcount, topiccount etc. You can get the chat messages from message objects and get users that did send at least one message from there to exclude them from the deletion.