Open Steam and find FaceRig in your library. Right-click on it and choose Manage > Browse Local Files.
Open FaceRig/mod/VP/PC_CustomData/Object/accesories.
Delete the file with the prop name you want to delete.
It will be removed from the prop list in FaceRig.
If you want to delete all the custom props you have added, you can also delete the accesories file itself.