Keeping up with the deadheading can be a challenge on these larger shrubs since they flower so well and the dead flowers are unsightly. To avoid the problem of self-sowing, try to deadhead consistently all summer not letting the seed to set.
Butterfly bushes are considered invasive in some states (Oregon, Washington, and Hawaii), so care should be given to not let them spread into the wild. Where they're native, they're a pioneer species that moves into open, sunny disturbed lands. They can set millions of seeds in one season. In the wild, the plants eventually get crowded out by taller growing shrubs and trees. In the garden, these seeds can be a problem without these natural controls in place. While deadheading will help, growing varieties such as 'Blue Chip' and some species types such as Buddleia fallowiana that have sterile seed or low-viability seed, will reduce the threat. Butterfly bushes are less weedy in colder climates.