If the spam is being sent by bots closing down 25 wont do anything becuase they dont send on that port..
Well, perhaps it wasn't worded too clearly. Any firewall, including the ISP's, can be configured to block traffic that is *calling* any port. So the idea is to block traffic that is calling on port 25 outside the ISP's own network, forcing users to route all messages through the ISP's server.