This depends on the type of bonsai that you have. For bonsai with thicker leaves, you can gently wipe the leaves with a microfiber cloth moistened with distilled water. While some sources recommend acids such as vinegar or lemon juice, high levels could burn your bonsai plant. If you are able to in the future, avoid spraying water on the leaves. You can also spray distilled water on the leaves with a spray bottle until it runs off to rinse it in the future. Distilled water can be purchased in the water section at most grocery stores. Avoid spring water as it has minerals that will leave further spotting.