Sure, there may be nothing legally, or even morally or ethically wrong (arguable), with just leaving and tuning out beyond office hours as a trainee. But the fact remains that it reflects badly on the trainee, especially when the justification is "I don't plan to stay or be retained". Just do it at your own risk because while most employers won't block your call, the legal circle is small and people talk.

I'm not saying trainees should be exploited, I'm saying try to be smart about how you handle a shitty situation, for your own sake if not anyone else's.