I got a bonus packet of variegated Nicandra last year, sowed them, and got a couple of wimpy plants that bloomed minimally, disappeared with frost, and I don't see any seedlings. Maybe the species is tougher.

I think knowing that it's invasive and a heavy reseeder is the first step to being able to use it smartly in your gardens. By staying on top of deadheading the plant, you can prevent it from spreading beyond where you want it. Just let enough go to seed for what you need then keep on top of the rest so as not to add to the problem. The ones causing the problems are the ones that don't take the time to find out about such things and just plant with reckless abandon.

Here is a link that might be useful: My Gardens