FTP termination by server in receive mode

Hi to who ever can help!!

I am busy writing a FTP engine for a PIC micro-controller using C. The MCU has to receive encrypted files via the Ethernet, save them on an external flash, decrypt them and so on… The application works very well… My only problem is that when the MCU recognizes that the file is not “meant” for it (by decrypting the first byte and comparing it to a saved value) it has to terminate the file transmission and not receive the whole 512KB file… How do I do it elegantly without getting the user terminal stuck? (I tried to give a command to close the port but then the user terminal just tries to resend the packets). Can I also send a message back saying, “The file is corrupted” or something similar?

Thanks, Noam.