Check out this article:
http://www.clemson.edu/extension/hgic/plants/landscape/trees/hgic1009.html
In particular:
When a large crape myrtle is in a spot where a low, compact plant is desired, there are two options: (1) dig up the offending specimen and then plant a dwarf or smaller cultivar that requires little to no maintenance, or (2) prune the stems back to about six inches above the ground each year. Severe pruning will not kill or injure a healthy crape myrtle.
Thank you very much!