Either buy despined prickly pear, or while wearing thick gloves, carefully remove spines from harvested prickly pears with a vegetable scrubber. Cut off both ends of fruit and remove skin. Chop the fruit roughly, combine with water and sugar, and bring to a boil. Use a strainer to separate the seeds and pulp from the syrup. Store the syrup in a glass jar in the freezer. Adding a little vodka keeps the syrup from the freezing solid without significantly changing the character of the drink you make with it.