It depends on the OS you are running. If you are running Vista/Windows 7 you can find the file under C:\Users\[USER_NAME]\AppData\Roaming\Leo Vildosola\Code Snippet plugin for Windows Live Writer. In there you will find a file called CodeSnippet.dll.config. Open it and remove the element Style. It will be regenerated next time you open the plugin.
If you are running under another Windows OS flavour you will find the file under C:\Documents and Settings\[USER_NAME]\AppData\Leo Vildosola\Code Snippet plugin for Windows Live Writer. If not there, try looking under C:\Documents and Settings\[USER_NAME]\LocalSettings\AppData\Leo Vildosola\Code Snippet plugin for Windows Live Writer.