Squid will help you control traffic through protocols it supports. For true bandwidth throttling or other traffic shaping use the QoS features in altq like stickman suggested. This way you can control the matter on your firewall, which is the logical location to do this anyway since your connection likely terminates there or nearby.
The PF user's guide will teach you all you need to know about using altq integrated with pf to do this.:
http://www.openbsd.org/faq/pf/index.html