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Hit the 'Stop' or Cancel button ans wait. The process stops when the program "sees" the stop button is pushed, and is able to processes it. The program will only process the stop when it safe to do so. As another safeguard, first the file is completely moved and then if all is well the MFT is updated to reflect the relocation.
This may take a long time depending on how much free space is available on the hard disk.
Do NOT turn OFF the computer while defrag is in progress.