Puree the watermelon in a food processor in 2-cup batches. You should have about 4 cups of puree.Press the puree through a strainer to remove fibrous material and seeds (even seedless watermelon has some seeds).
Transfer the juice to a 10-inch nonstick skillet, bring it to a boil over medium heat and then reduce the heat and simmer until thick, about 1 1/2 hours, stirring from time to time. Watch carefully toward the end to prevent scorching.
During the last five minutes of cooking, stir in the vinegar, mace, cloves and cinnamon. Taste and add more spices if you like.