Depending upon how heavy the infestation is, you have several control options. Aphids can be knocked off with a stream of water, or picked off for those less squeamish. Controlling aphids organically there are several options. Lady bugs, lacewings, and parasitic wasps are natural enemies. These beneficial insects can be purchased at local garden centers and released in the garden to help reduce the problem.
Insecticidal soap can also be effective in controlling aphids.
Chemically there are several products that can be used such as Malathion, Diazinon, or Sevin. Be sure to read and follow package instructions and precautions when using any of these garden products whether organic or chemical compounds.