What is the best way to remove and prevent leaf stains on concrete pavers?
The tannins from oak leaves will leave stains behind. I would suggest not worrying about it. Those stains are just character, part of nature. Choose to appreciate nature.

You could probably bleach them out (using simple chlorine bleach or oxalic acid), but why? Why introduce harmful chemicals into the area that may hurt you or damage your plants? Besides, the bleach may not get everything consistently the same color as it was. So you may just leave yourself with another problem to solve.

Sunlight will cause them to fade over time anyway, and as other stains occur, the pavers will become randomly stained, eventually all to the same color.