If you have a good nursery nearby, ask them if they carry live Lady bugs or Praying Mantis Eggs. They'll both do great at cleaning up your aphids as well as other nasty insects. It's a seasonal item, they may not have them this time of year, but worth a try. You may also buy them online.
Personally I use a soft brush. I've been doing it to my new apple tree saplings this year as the aphids have been particularly bad.
Use a soft brush to literally brush them from the leaves. If its a pot plant indoors just take it outside to do it.
I use a soft broom head but quite easily an old toothbrush or a shoe brush (my family still polishes shoes the old fashioned way) or other similar brush is just as good. Simply brush them all off as much of the plant as you can, repeat many days later if necessarily. It can actually be less time consuming than you imagine and is very effective if like me you don't have an outdoor hosepipe
Water. Like Yvonne said. For now. They can easily be spray streamed off while you water.
For future, consider planting plants that attract ladybugs into your garden and if you can get double duty like DANDELIONS (!) which attract lady bugs and are tortoise edible, even better. Also for future, build up your soil under that hibiscus. Weak plants attract the enemy. LOL. If you have a strong soil system, the pests go elsewhere. Law of nature, survival of the fittest. For hibiscus, I like chicken manure (I get it at Home Depot, bagged and already composted) and coffee grounds from the Star___ coffee place. They give for free but most any donut/coffee place can be talked into a grounds for this gardener please plan. Just strike a deal with the owner or manager. Most are happy to have you take them. Also, I findthat with chicken manure as a supplement I get no white flies. I think it makes the hibiscus less attractive to them, taste wise. Yay.
Hibiscus like their soil a bit acidic and coffee grounds is a good way to do it. And after a long day, if you soak your feet in epsom salts throw the water out into your hibiscus. Epsom salts is magnesium sulfate and excellent for plants as it strengthens the cell walls and really helps in the uptake and therefore use of nitrogen, phosphorus and sulphur. Roses love Epsom salts also and tortoises love roses!
Stronger hibiscus and roses, more flowers for tortoises, less aphids.
Here are some companion plants, for starters as there are many more, that will attract ladybugs into your garden: butterfly weed, dill, fennel, golden marguerite, Queen Anne's lace, coriander/cilantro, buckwheat ... and as mentioned before dandelion, lotsa dandelion. : )
Mix a little bit of dish soap ( dawn is what i use), and about a table spoon of cooking oil in a spray bottle , fill the rest with water and spray. The soap will break down their bodies and the oil will help it stick to them. You can goggle on YouTube to find organic ways to get rid of bugs in your garden. Check it out. YouTube has helped me in making my garden pesticide free.
Thanks everyone! I'm going to try the dish soap and oil until I buy some dandelion seeds. Hopefully I'll get some ladybugs to help me out. Our soil here is so bad and clay like I'm also going to try adding coffee grounds to it.
We've had more than one mention of earthworm castings doing the trick. Apparently something in the earthworm castings helps the hibiscus fight off aphids. I have not tried this myself, but others members swore by it.