When you want to put something else in their place Nanamorb. Just leave one to seed maybe. Pot up seedlings to grow on. Then you can have them where you want to plant out later.

Are they perennial wallflowers, Nanamorb, or annual ones? If they're perennial, they'll probably have small shoots appearing towards the bottom of the main stems, so you could cut them down to just above those, and they'll bush out again eventually. I'd cut off all the seed heads too, as Dorjac says, but leave one on to self seed. If they're annuals, just save the seeds for next year when ripe, and take the plants out. Annie