physical port locks for ethernet, USB, phone, etc.?
Solution 1:
If it's just the fact that the port is empty, you could buy a bunch of RJ45 connectors (or alternatively cut them off the end of old patch leads) and clip them into the ports.
If access is still an issue you could trim the end of the clip so that you'd need a small screwdriver to operate the release.
Solution 2:
RJLockdown is the only product I've been able to find that seems to be able to do so. Its a bit ugly, but it should do the job. Most solutions entail MAC filtering and software solutions, which obviously is no good for your situation.