If I see port 25 is open, but I dont have access to the program running the smtp server, could It be flooded with enough information to make it unusable (but not affect any other services)?
You could make the program behind the port unusable. If the given program handles the DoS so badly that it actually fully crashes and thus dies, the program will give up the port, and the OS will reclaim it after a moment or two. After that, it's available for use for any application running on the machine. Assuming it has the right priviledge to use that port and so forth.
Opened or Listening Port means a service working and listening to this port(s) in order to do something.
Like Web server, it's listening on port 80 which mean a particular service listens to this port
to stop the port you have to stop the service it self. You can stop the service by kill its process or Attack it by DoS (if it's vulnerable) or DDoS by flooding this service.
I asked this question without a good understanding of ports, however, I now do.
A port is not a fixed bandwidth like a usb plug or cat5 connection, just a way to tell the kernel what application is to be reached.
It is therefor not possible to flood a specific port and prevent access to only that port in the way ddos does to an entire server