Your iSeries 400 can have a lot of incoming TCP/IP connections coming from different machines, it can be another iSeries, a Unix machine, an NT machine or a mainframe.
When one of the machines connecting to this iSeries encounters TCP/IP problems, you cannot end the connection normally. You need to end it abnormally.
How do you do it?
First know the IP address of the remote machine that had encountered TCP/IP problems.
Then, run netstat *CNN, it will give you a list of all connections on the iSeries machine. You will see a lot of IP addresses, you can page down but you can also press F17 (shift F17 on a PC keyboard) and enter the IP address of the other machine you are having problems with, it will bring you there, and from there, you can put a 4 to end the TCP/IP connection with that machine. Be careful on doing this because you can have several connections, this should also identify the port giving you more information on what to connection to end (you can have several entries with the same IP address but they will have different port assignments). Usually the ones giving problems are stuck on a close-wait state, but I still suggest verify thrice before ending things.