This is how QuarkXPress running in a QPS environment is designed to work. You can use the following workaround if a user forces a Check-in from another machine:

  1. Make a backup of the checked out file from machine A.

  2. Override the Check-out of the file from machine B.

  3. Delete the local file from machine B.

  4. Copy the backup file of machine A to local folder on machine B.

  5. Check in the file.

For files that have been checked out, the system will store the machine host name in the Check-out Machine Name attribute.
It does not store the IP Address of the machine in any attribute due to the possibility that it could change in a DHCP environment.
The behavior is designed in such a way that it verifies the Machine Name, QPS User and Application during open/checkout and while logged into QPS. Therefore it will not allow a user to check in an article checked out on another machine, even if logged in as the original user on the second machine.

Additional Notes:
You can also rename Machine B with the same name as Machine A.