Support for SermonSpeaker 4.x is canceled as it is for Joomla 2.5. I will gladly assist you with minor issues but I will not fix any bugs anymore in this releases.
The fix is to upgrade to SermonSpeaker 5.x and Joomla 3.x, which is better anyway.
It was created because someone followed a series download link. The file is only created on request, so either you had the series download column activated one time or someone just tried blindly.
Then disable it
Starting with SermonSpeaker 4.4.7 where the series download got improved you have now several ways to restrict the series download. You can do so for all series in the SermonSpeaker options (Global tab) or you can disable it for single series when you edit this series. You can also limit it based on how many sermons the series contains. Prior to SermonSpeaker 4.4.7 you could have put a file named 'stop.txt' into the series subfolder of your sermon path. This still works, but the settings does the same job.