As I previously posted (as an employer) I think the op is a bit of a knob in this situation. But we don't have all the facts.
If the employer is constantly having short term employees then they are probably doing something wrong too.
Are they paying way below market pay rates?
Are they treating their employees like excrement?
Other?
What they are doing wrong is (imho) employing people who don't have all the necessary skills and needing to train them without any recourse. They probably have the noblest of intentions, to help people better themselves and they are getting bitten on the proverbial.
I would suggest that once these people are gaining the qualification from the training, they are moving to greener pastures.