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 support@team.getblend.com as soon as possible. You will only need to pay for the words translated up until the stop request.