How did you try to remove it? Also there are more than one place an extension can live/hide. I would check /System & /Library.
You can use kextunload like this...
sudo kextunload /System/Library/Extensions/extension_name_here.kext
Or at the filesystem level
mount -uw / rm -rf /System/Library/Extensions/extension_name_here.kext or rm -rf /Library/Extensions/extension_name_here.kext
If after doing these are you still seeing the extension loaded?