Have you tried pinching them back? It's usually done to increase branching and bloom, but it also keeps the plant shorter.
I think the rule of thumb is keep pinching until after the fourth of july, then stop and wait for blooms.... and don't just pinch out the growing tip, if you want shorter plants, chop it down to three or four inches in July and let it branch from there.
I bet there are short types out there, I don't know which ones they are.
hope someone else has better advice!
Kato