Try mulching with coffee grounds or worm castings as an organic approach, but it may boil down to the plants are struggling with less than ideal conditions and so more subject to attack. A systemic insecticide might also work, but could take a toll on the bees and hummingbirds, so I prefer to avoid them with flowering plants that are bee/bird magnets. It might just be easier to replace them with another type of more resistant plant, rather than fight a continual battle.

How many years has this been going on? It took me two years of squishing to get rid of stink bugs, but it finally reduced the population enough for me to eliminate them (knock wood). If it has been more than two seasons, I would consider using a systemic or getting rid of the plants.
Renee

tried systemic but it only works for a little while and I am concerned about my cats and the birds who frequent the garden. The cestrum leaves are very tender and sort of invite nibbling...so it might be time to try another plant