How Can I Mention The Download Limit Of 5mb In Broadband

I Have the setup of 100 computers. All of are sharing broadband connection. My problem is that i have the subscription of 10 gb download and it exceed to 30gb. How can i overcome this problem. is there any software with the help of i assign the download limit of x mb to all the user. Pl help me if some can.

Parvinder , check this : netlimiter
similar softwares are available on net . pls search the net .