Sure you can prune your Annabelle hydrangeas. Except, do not prune in the spring or early summer. Annabelle hydrangeas bloom on new wood on stems that have begun growing in the spring, so if you prune the new growth before they bloom, it will be the following year before you see a blossom.

Annabelle hydrangeas can be pruned back to about 3 inches above the  ground in the winter. In fact, many growers cut them back to get a more uniform look instead of having unattractive branches to look at during the winter. Annabelle's will bloom every year, no matter how they are pruned or not pruned.