Appartment internet sharing issues

Our apartment have 20 flats and having normal 24 port switch and 4 Mbps ADSL, 80 GB limited internet line. After 80 GB, the speed will downgrade to 512 Kbps. As some of peoples are using torrent downloads most of tenants are complaining about the speed. My requirement is, i want to fix bandwidth for each flat, 1 Mbps or 2 Mbps fixed. what is the best solution…??? any devices are available to control and distribute the internet speed bandwidth equally or as per administrator’s policy..??