There are few options to control Network, or shared Internet activities on other Network computers.
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.
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 does not belong to you (like in a Dorm Network), people have to agree to put them on, and any well-informed person can always disable this software.
Before any, one asks. ?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.