Citrus are pretty tough when it comes to aphids. Greenfield Citrus Nursery says there is no need to spray Citrus for pests. The one exception may be San Jose Scale not aphids. However if you feel you must do something simply spraying them with water is enough to knock the little buggers off and kill most of them in the process.

Please don't use insecticides or soap spray for aphid control. You'll be killing pollinators like honey bees and natural predators if you do. As thisisme mentioned, citrus are tough. Try a little patience and soon ladybeetles, lacewings and other predators will arrive to take care of them for you.

Here's a link to aphid control from UC Davis.

Here is a link that might be useful: Aphid control

There something you can do if ants are part of the problem. You see ants protect aphids from their predators. They do it because aphids secrete sugar in their feces and the ants eat the sugar. You can buy a product called Tanglefoot and put it around the tree about 12" from the base of the trunk. This will keep the ants from being able to climb the tree to protect the aphids which allows the aphids predators to live in the tree and eat the aphids.

I agree, don't spray. Especially in a hot environment, aphid is usually a minor pest and does not require control.
If you have sprinklers spraying water on your citrus, you may want to correct that. Also other wet areas that harbor insects like aphid, thus creating a supply of the critters.