What RockyMtnMan used would also work. Also good advice to really drench them. Sometimes I would drench them, wait an hour, drench again...
I used a rosemary oil, garlic, peppers, soap mix trying the organic approach and thought I had them beat for 1 1/2 weeks. They came back even stronger.
If you use rosemary oil try to make your own, find a rosemary bush(grows everywhere where I am) take about 2-3 oz of rosemary, stems and all, and cook them in a slow cooker just covered with oil smashing them from time to time for about 2-3 hours. This will make a very strong rosemary oil and will last a long time. I use it now mostly as a repellant/deterrent.