Unfortunately you need to invest in an ice cream/sorbet/gelato maker for this. I use a Cusinart and it works fine for me cause I have an extra fridge/freezer. That helps a LOT.
I got this recipe off of the book ‘Vegan Scoop’
1/2 cup of agave nectar ( or a little less if you don’t want it too sweet)
3 fresh mint sprigs ( I used a little more)
1 1/2 tablespoons (23 ml) of fresh lime juice
2 tablespoons of light (Bicardi) rum ( I used about 2 shots—3 oz)
1 quart of water