Yes and no. Yes in theory, using bandwidth-per-source-IP caps like you suggest could be used to solve this. No in practice, because the prevalence of NATs put

Yes and no. Yes in theory, using bandwidth-per-source-IP caps like you suggest could be used to solve this. No in practice, because the prevalence of NATs puts many users behind the same IP address, meaning that unfair users can still hog the upstream bandwidth from everyone else in the same NAT. What we really want is something more like bandwidth-per-end-host caps.