Luckily, there's no need to destroy your lovely plants. But you do need to destroy the seed. Grab your favorite pair of pruners and give your fountain grass a haircut soon after the flowering plumes emerge in late summer. By removing the flowers, you are removing the seeds and eliminating the chance of seedlings next spring. Also, dwarf fountain grass cultivars are reputed to be less invasive than full-size selections. Look for 12-inch-tall 'Little Bunny', for example.