It's a sticky job, but just pull (or snip) off the dead flowers.

You need to nip off the developing seed pods, not just the dead flowers. Once the plant has developed ripe seeds its job is done for the year and it dies. You can keep annuals flowering by removing the seed pods so it has another go and produces more flowers.

Sorry -I should have clarified better.

nopody is ferpect

;-)

To clarify more, don't just pull off the colored part of the flower, but also get the green, star shaped part that they are attached to. It may make little difference this late in the summer, in the UK, but something to remember next year.