Are you sure you sure you don't want to make a layer mask instead?
This is for a stock website. They are very strict with what they accept. They don't want any masks, only straight vector graphics that can be opened in Illustrator 8. A huge pain in the butt.
Then you will need to cut and trim everything back to the size you need
That's why I wanted to use the crop tool from pathfinder to avoid all the manual work.
What happens if you create the rectangle and Object>Path>Divide Obejcts Below (and see what is left to cut, maybe open paths which may have new Anchor Points at the intersection with the rectangle which may be cut with the Scissors Tool)?
For some reason, it removes elements from within the artboard
The reason depends on specifically how your paths are drawn.
You should be able to do this:
1. Position a horizontal guide at the bottom of the Artboard.
2. Knife Tool: Position cursor outboard of Artboard. Press Alt. Mousedown (snapping to the Guide). ShiftDrag rightward across the bottom of the artwork.
However, that assumes reliable and accurate Snap behavior across the toolset, which Illustrator's isn't.
To avoid figuring out all the caveats of Illustrators very poorly implemented boolean path combination features (so-called Pathfinders):
1. Rectangle Tool. Draw a rectangle outboard of the Artboard, snapping the top of it to the bottom of the Artboard.
3. ShiftClick one of the paths you want to cut.
4. Click the "punch" (Subtract) Pathfinder (assuming CS5 or CS4; altClick the same Pathfinder in CS3 and prior.)
5. PasteInFront. Repeat steps 3 & 4.
Your second suggestion is what I did, which didn't work. I'll try the first and let you know. Thanks so much for the suggestions!
After completely taking apart this file, it turns out I had a "gradient mesh" that wasn't really a gradient mesh. It was a gradient mesh, but I removed the points and just filled the shape with a solid colour. The problem is, once you create a gradient mesh, it remains as a gradient mesh, even if you only use one colour in your shape. Once I recreated the shape and got rid of the gradient mesh, it was fine. Also, I used the eraser tool, which got rid of areas outside of the artboard.
Thanks for everyone's help. Hopefully this will help others having the same problem.