Either way. Extending the light period long enough to stop the development of the flowering hormones by putting light on each plant for a couple hours will do it. But putting a couple of lights in the yard will not cover each plant. You have to keep in mind that each plant will have to be treated the same as if it was in a tent. Putting a 4 tube Fluorescent over one plant will work for that one but not do anything for the plants that are several feet away. I cannot think of a way you can get the extra light to all those outdoor plants in an easy way.
I get the feeling that you are stuck with letting the seasonal increase in light do the job of causing the plants to slowly go back into their vegetating stage until mid August or so when the longer nights start to cause flowering to start over again.
