There is no way to enforce an IP/username combo in order to allow web browsing. The SWG is not designed to be a device to keep people off the web, it is designed to keep them safe by scanning content they access and download.
The incident history takes up a lot of space in the database and it is not designed to hold more than 10 million events (Depending on your hardware. If you are running on a virtual machine, the limit is 500k). Depending on your user's activity, this limit is usually reached in a month or two.
You can always export the reports weekly. They are in CSV format, so they are easily parsed.