If you have sprouts down low you can prune the top. The best time to prune is just as they finish blooming and before they create seed pods.

I have never heard the term "sun azalea" before. I googled it and found that it is what I call the Formosa Azaleas or similar Southern Indica Azaleas. It is an evergreen azalea that is not very cold hardy. It is very heat resistant and sun tolerant. They are vigorous, upright growers, often reaching a height of 10 feet or more. That means that you will have to keep at it with the pruning if you want it to fill out. It's natural habit is tall and straggly.

Here is a link that might be useful: Sun Azalea