Unfortunately, the only way to reduce the size is by dividing the clump. You could try taking it back along the perimeter of the clump just by root pruning a few inches in with a sharp bladed shovel but I'd think that more work than dividing. And with pampas grass, you would definitely want to do either activity after the grass was cut back hard.

I wouldn't advise using a border - it would only work for a season or two and then you'd have to go through the process of dividing anyway. Most ornamental grasses expand on their exterior edges and die off on their interior. That's why they need to be divided periodically.

Make the flower bed bigger ; )