The jpeg (EXIF) thumbnail is stored in ImageEnView.IO.Params.EXIF_Bitmap, so you can resize or remove it just before saving.
For example, if you want a 300x### (not 300x300 to avoid wrong stretching) just execute:
ImageEnView1.IO.LoadFromFile('input.jpg');
ImageEnView1.Proc.ResampleTo(ImageEnView1.IO.Params.EXIF_Bitmap, 300, -1, rfTriangle);
imageenview1.IO.SaveToFile('output.jpg');
To remove EXIF thumbnail:
ImageEnView1.IO.LoadFromFile('input.jpg');
ImageEnView1.IO.Params.EXIF_Bitmap.Free;
ImageEnView1.IO.Params.EXIF_Bitmap := nil;
imageenview1.IO.SaveToFile('output.jpg');