They can be pruned when all the leaves have fallen. The only branches that should be cut are branches that are too long, branches that may be rotted or diseased or branches at the bottom if there are too many for your liking. There is no need to top the crapemyrtles or to “hat rack” them. It is not healthy for the tree to be severely cut back. This weakens the new branches that come out and could cause diseases and insects to enter the wounds. If you are looking for full flowering, cut the dead flowers after they are spent to promote further bloom production 

Additional Resources

http://edis.ifas.ufl.edu/MG266

http://pcetimelytopics.blogspot.com/2008/01/pruning-crape-myrtles.html