In order to limit the amount of data downloaded by user or group you can use the “quota” rule. Go to user or group profile page and in the “rules and restrictions” tab choose “add > quota”. While creating a quota you need to define daily, weekly and monthly limit of the downloadable data; you can also specify resource, protocol and port which will be accounted for in quota.
If you specify any setting with «0» - the quota will not be applied.
Warning: Lowest quota takes priority when several quotas specified for same resource.
If a user exceeds the quota his internet access will be blocked and he will only be able to access the resources for which there is an allow rule created with the option “allow always”. Aside from blocking the access to a particular resource, you can also limit the access speed to that resource.