There are a few chemicals you can use to remove sap from your pavers. I recommend using WD40 or Isopropyl alcohol, these two chemicals break down the tree saps composition allowing you to use very hot soapy water in combination with a stiff bristle brush. Please make sure to give the chemicals enough time to work before using the hot soapy water. Several applications will be required for tougher stains.
For stubborn tree stains, you can use paint thinner in combination with hot soapy water and a brush. However, please be warned that using this chemical produces a bleaching effect and will wash away the colour in your pavers. I suggest testing it in a hidden spot before you try it on an actual stain.