Some modems have custom firmware that let you limit it via the modem itself.
Otherwise, if you have a server you could set up internet sharing through the server and install a program such as Bandwidth Manager on it to limit and monitor it that way.
Internet ---> (Network Card 1) Server (Network Card 2) ---> Network PCs.
This would let you set it all from the server side. http://www.antamedia.com/bandwidth-manager/
I wouldn't recommend installing limiting software on each system as it's possible to get around it.