"Remove" from where? They're not actually in Lightroom, just referenced in the Lr catalogue.
I guess I meant how do I stop all the previously imported photos from still appearing in LR? Still, how do I just import 1 photo instead of the lot or a full folder?
Importing just one photo is not the idea of using Lightroom. Instead it manages pictures - note the plural - and applies adjustments to them. Once you record or "import" pictures into Lightroom, you keep them recorded in LR, and ideally you record all your pictures. It's not intended for opening and closing files like you would do with Photoshop.
You should go through the Getting Started material (link at top of page).
I guess I meant how do I stop all the previously imported photos from still appearing in LR?
Well, assuming we're talking about manageable numbers, you can right click on the thumbnail and select delete from there (you can either delete the file completely, or just - in effect hide it); but personally I wouldn't bother: they do no harm being visible, and - if you only want to see a single image (picking up on John's advice) - you could place it in its own collection; or "filter out" the unwanted images; or add the lone file to a different catalog.
But none of those ideas (and there are probably more) are how Lr is really intended to be used, in broad terms.
But all of this doesn't answer the question that has been asked. If you only want to import one image, choose the folder that contains the image that you want to import. By default, Lightroom is going to offer to import all of them. But you can uncheck all, and then check the one image that you want to import. That will be the only image that will be imported. As others have explained, this isn't the most efficient way to utilize Lightroom. But if that's what you want to do, that is how you do it. The uncheck all button is near the bottom left-hand corner of the import screen.
I have a similar question. I have LR4 and just starting to use it. When I was uploading photos, somehow they were loaded into LR twice. I manually deleted one set but now the one set left in LR, when I try to open a photo, I get an error message: "The file named "IMG_7432-2.CR2" is offline or missing". I have not made any adjustments to any of the photos in the Catalog. I would like to delete the whole catalog and start over. I thought all I had to do was drag the catalog to the Recycle bin. However, other posts make comment about an .lrcat file.
**To enhance performance, Lightroom builds a Previews folder next to the .lrcat file in the Explorer (Windows) or Finder (Mac OS). It is safe to delete this folder as long as its name matches the .lrcat file name. If you delete a Previews folder that’s still needed by a catalog, Lightroom will regenerate it when you work in that catalog, but it will run more slowly until the previews are rebuilt.**
Since I want to delete the entire catalog and start uploading all over again, do I have to worry about the Previews and the .lrcat files? If I upload again, do I have to do something to rebuild the previews?
Thanks for your help!