From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1896-745198
Question: Help! My mums are already blooming! Should I cut them back?
If I do cut them back will they bloom again in the fall?
Answer: Mums are typically pinched back repeatedly, beginning when
they are only about four inches high and continuing until about the
fourth of July. This procedure delays their flowering until fall and
at the same time makes them dense and bushy. In some cases, it is
possible to enjoy their spring bloom and then cut them back and
fertilize them to encourage a second flush of bloom in the fall. Since
they have already devoted energy to forming the buds, you might as
well enjoy the blooms now.