There would be a process that is listening on that port. Stopping that would be the first order of business.

It might also be that inetd is listening on that port and will fire up a process as specified in /etc/inetd.conf (does the name stay the same in AIX?).
In that case, find what process it is (usually from /etc/services - again confirm the name for AIX), then hash out the line referring to that process from /etc/inetd.conf. Send SIGHUP to inetd. That should do it.

If there isn't a process listening at all - or rather, there was a process, but it was killed or stopped uncleanly and didn't release the port, then just give it a few minutes. Connections to the port will drop once they timeout.