Q: How can I prevent the massive leaf growth at the bottom of my crepe myrtles?
A: Severe winter pruning is almost always the problem.
When you remove the top of a crapemyrtle it automatically tries to make new growth at the base. You have to choose whether to stop winter pruning or prune away the basal sprouts each year.
You could try using a sucker control product (click for sources).
There’s also a possibility you have a “shrub-form” crapemyrtle that you’re trying to shape into a tree. No matter what you do, this type plant will have many sprouts at the base.