Except in the tropics, marigolds are annuals. They only last one season, but they will usually flower until killed by frost. If you live in an area that will experience frost very soon, you might want to just let the plant go.
You could wash the bugs off with a spray of water from the garden hose, or if the container is small enough to be handled easily, you could just wash the plant off under a faucet.
Another treatment would be to spray the bugs with a solution of 1 tsp pure soap dissolved in 1 qt of water. Be careful to use real soap, and not detergent. Detergent can burn the leaves of the plant.
If you decide to treat the bugs, be sure to spray or wash both sides of all the leaves and also the stems. You should repeat the treatment every 7 to 10 days to kill any new ones that hatch out.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams