Can I cancel the erase process?
Yes, sure, you can.
The Cancel button is active before the erase process goes to 100%, then ByebyeData Eraser will display the result: succeeded-failed-cancelled. What were "succeeded" cannot be recovered, so if you're counting on the Cancel button to get erased data back, you'll be disappointed.
Take the following screenshot for example, I'm erasing 11 files, when I cancel the process after the first item was erased, I can save the rest 10 files, but I'll never see the successfully erased file.
As for the Cancelling the Erase free space process, although the message box will show as the same – Cancelled:1, it's quite different when you cancel it at the 1% rate and 99% rate, data recovery software would still get most of the deleted data back when it's 1% erased, but if that's what you want for the cancel operation, you may cancel it as early as possible.
The same to the Erase entire disk and Erase entire drive process.