A yes, two weeks back the same thing happend to me! A mischevious child when and punched in the dust caps on one of my Sequerra montiors (no grills)---more than just quarter size...all the way. When I told my wife about it, he overheard and promply went and punched the other one in too as we watched dumbfounded.
Fortunately, I was angling for some new speakers so I told my wife that since this was her friends child, at least we could get new speakers. So I bought some Acoustic Zen Adagios. I feel that I made out pretty well!!
But if that's out of the question...well you may be able to remove the the dustcaps altogther and replace it with a phase plugs (make sure you can find the right size before doing that). (As i realized once I removed the dustcaps on my Sequerras. I found a website with some wooden ones here: http://www.planet10-hifi.com/pp-testimonials.html. I'll probably do that anyway with the Sequerras.