im having the same problem! ive done several different edits of the same shot and exported them with watermarks on them so that they couldnt be taken however, now that i sold an image i want to take the watermark off but cant!
If you have already exported to jpeg or tiff with a watermark you simply cannot get rid of it without some manipulation. Just make sure that for future exports to jpeg or tiff you do as suggested by so many others above, uncheck the options that watermark the image.
Like Rikk said, work from your original RAW image and export without a watermark.