How to control internet speed in network ?

I have net cafe.
I am facing one problem related to internet speed.
when multiple user using net, some of them start heavy downloads.
when download starts other PC's net speed decreases suddenly.
How to control it ?
Can I put some limitation on every PC,
Ex. each PC can consume only 50-60 KB speed not more than that, so other PC net speed will not affect.


Is there any free software ?

You can put a block on some of the sites where users download files, songs, etc from.