You have several solutions:
1. If your router supports qos, you can enable qos and limit bandwidth directly in your router.
2. If your router/switch supports "port mirroring", you can setup a mirroring port and do filtering on this mirroring port. Monitoring programs like "WFilter Enterprise", "websense" can enable you to monitor and filter internet activities of all network computers from a mirroring port.
3. Setup a bridge or gateway box to filter internet traffic and limit bandwidth.
4. Setup a squid proxy server.