Certified GKH Gardening Expert
You can, and the plant will survive. But it will seriously and negatively affect the shape of the tree. This kind of pruning is often called "Crepe Murder".
Do not commit crepe myrtle murder which is cutting off all branches at the same height!!! Only prune back branches that are toughing because the cross each other. If you must shorten the plant, you can cut it back but do it in a rounded fashion and be sure to train the plant when it begins to grow in spring by saving only two of the many branches that will occur. Be sure to pick ones that are heading out from the center of the tree