Unfortunately, there isn't a way to remove a source file from an active project.
If translation has NOT STARTED:
Simply cancel the project (click “Cancel” on the project page) and submit it as a new project with the correct file.
If translation HAS STARTED:
Post a comment to the discussion board on the project page asking the translator to stop work.
Contact email@example.com as soon as possible. You will only need to pay for the words translated up until the stop request.