Both sentences are past tense the difference is when they would be said. "You were warned" would more likely be said long after the instance of the warning being given. "You have been warned." is something that would be said almost immediately after the warning was given and in media it is usually said by the person who gave the warning. For example, if a fortune teller tells you to avoid large groups for 1 week and you say you're going to a huge party tomorrow the fortune teller might shrug and say "You have been warned." It is a very ominous statement suggesting that the warning might be ignored. Both of those sentences can be interchangeable it depends on the context.

you were warned: The "warning" was made clear in the past. "You were warned about not passing your homework." you have been warned:#1 Implication a warning in that exact moment. May also be used as a threat. "Pass your homework in on time or fail. You have been warned. #2 may also mean the same as "you were warned" depending on sentence context. "you have been warned several times to turn in your assignment!" I hope this is helpful =)

"You were warned" is a phrase that explains why a bad consequence has happened to a person who has done something they were warned not to do. It is only used after the consequence has actually happened. "You have been warned" is a statement that emphasizes a warning immediately after the warning is given. This statement is usually viewed as a threat, and is given before the consequence has actually happened.