Select the file, right click and select "rename". You only need to change the ".bgl" extension.
However, if you're not seeing the 3 letter extension, renaming won't help.
In that case, I can think of 3 options:
1. Change your folder options so you can see the 3 letter extension, then rename. If you don't know how to do this, Google "Show file extensions" and choose the version of windows you are using to get instructions.
2. Add a new folder to your KLAS scenery folder called "old". Then just move the suspect files into it. That will hide them so that FS can't see them.
3. Just delete the suspect files.
Note: If you don't have Taxi2Gate KSTL installed, you should probably keep the KSTL library file.