Assassin bugs are considered a truly beneficial insect. Just leave 'em alone. They are there to help you by eating aphids.
Often you'll see one or two of these amazing creatures on a plant where there are also plenty of tiny, yellow aphids.
There is an interesting dichotomy, however, when it comes to controlling aphids with assassin bugs alone. When you see an assassin bug or two and there is a huge infestation of aphids, the bugs likely won't be able to totally control the pests.
So, do you try to kill the aphids and risk killing the assassin bugs in the process? NO!
The best thing to do is pick off the assassin bugs and move them to another plant. Then treat for the aphids on the plant in question.
You should never kill assassin bugs just because your aphid infestation is really bad.
Any liquid insecticide will work on the aphids, as will soapy water or even a water blast from the garden hose. If you use soapy water, please rinse the plant off with plain water a couple of hours later, so the soapy film doesn't interfere with the respiration process.
If a blast of water alone is your choice, be warned that the aphids will find their way back to the plant, usually within 48 hours.