If you want to see it flower, then the answer is Yes...it is too late.
If you don't care about the flowering and want to reduce its size, then prune away....

Ian

Another reason to consider pruning now would be if you had problems with Viburnun beetles this past year. If you did, then the ends of the branches will be full of tiny holes where they laid their eggs. Cutting them off will greatly reduce the problem next year.

If you did not have a beetle problem, then you should leave them alone and prune next year after they've finished flowering. The flower buds for next year have been set already and if you cut them off you will have to wait until the following year to see any flowers.

BP