There are few options to control Network, or shared Internet activities on other Network computers.
1. The best is to buy a Switch that is manageable by software residing on the server.
Do not run to look for such a Switch, unless you are ready to spend big money, entry-level hardware cannot do it.
2. A simpler level of control is to put Network managing software on the main computer, and client managing software on the rest of the Network computers.
If the other computers do not belong to you (like in a Dorm Network), people have to agree to put them on. Take into consideration that any well-informed person can always disable this software.
I am not aware of freeware software that does this. However, there is few low cost shareware available for this purpose.
Search the shareware sites for term like Port blocking, Network Bandwidth etc.
3. You can eliminate your current Routing system, and build a Linux Box. Linux software (free) has a capacity to Route, and control distribution to other computers by software setting.
Linux Box ? is the term used to describe a dedicated computer, with to two NICs running Linux operating system, which is connected to a switch for Network distribution. Such a computer is performing similar function as the Cable/DSL Hardware Routers.
Example of such software:
Free Software for Routing.