I cut ours back in spring before it starts to grow. Be as drastic as you like.

If it is only blooming once in the spring, it may be a species that should be pruned in late spring right after it blooms.

Steragram, can't really wait till next spring.
Tugbrethil, It has bloomed and is now all dead blooms. Maybe I'll just lop it all off.

A neighbour chopped his right down in summer and it did recover. Hard to kill them really.

If it had spikes of flowers at the tips of the stems, it is probably a variety or hybrid of B. davidii. They can be cut back almost any time, since they bloom on new wood. In fact, if you cut it back moderately now, it may bloom again before fall. A concern is cutting it back too late, so the new growth is damaged by frost.
If it had short, roundish spikes all along the twigs left from last year, it is probably a variety of B. alternifolia. That species should be pruned as little as possible right now, since the flower buds for next year are forming right now. If you have to prune, it won't harm it much, but you may not get bloom next year.
Since it just finished bloom this late, it sounds like it is the former.