Blocking DHCP between 2 networks

Hi there, is there a way I can block DHCP being transferred between my networks? Let me explain the setup and idea:

There are 3 factors here... the new MAC network (self contained using DHCP from OS X SERVER), a MAC PRO (2 ethernet ports in a remote location but hard wired via ethernet to the MAC Network and WINDOWS network) and an existing WINDOWS network

I have created a MAC OS X Server network for a department to be run completely separately from the existing WINDOWS network (different logins, storage space etc) The MAC network is up and running using DHCP, all home folders are hosted on the server allowing users to login to different machines (classroom environment). Works perfectly until I try share the internet out from the windows network...

I want to port the internet through from the windows network to the mac network just for internet access nothing more. I have a remote MAC PRO in the recording studio which currently has internet access from the existing network and I want to use this to share the internet connection to the MAC network as well as give it file sharing access etc. This is what I've tried and what goes wrong...

The MAC PRO has 2 ethernet ports, 1 is connected to the WINDOWS network (MAC PRO recieves IP via DHCP and it is possible to login as a windows user on the MAC PRO), when I disconnect the windows network and connect the mac network (IP via DHCP server in the MAC suite) I can operate on the MAC network with no problems at all.

PROBLEM: When I connect to BOTH ethernet ports using the MAC PRO, the DHCP of the WINDOWS server takes priority and assigns the IP addresses for both cards which then in turn pulls through to the MAC network and all the machines in there start looking to the WINDOWS server for login. Disconnecting the MAC PRO from the MAC network and reconfiguring the DHCP lease on each machine is the only way to get them back working again...

I've read that blocking ports 67 and 68 should stop the DHCP but I've no idea about how to tackle this on the MAC PRO if I'm honest.

I've also been advised to use squid but again not sure how to implement this. My line of thinking is that asking for a static IP address from the network manager for Windows server will stop the DHCP transfer and solve the problems? or will the Mac network still look for it through the mac pros connections?

Strangely, when I make the ethernet port on the MAC pro that is connected to the WINDOWS network inactive the problems still arise and the windows DHCP still gets pulled through to the MAC network?!

Apologies if this is garbled, I have limited knowledge but quick at learning, only just self-taught how to set up the mac network just need a point in the right direction that's all and I'm sure I'll get it sorted

Can anybody help?! Thanks
Chris