disappointed: upset because sth you hoped for has not happened or been as good, successful, etc. as you expected.
- They were bitterly disappointed at the result of the game.
- I was disappointed by the quality of the wine.
- I’m disappointed in you—I really thought I could trust you!
- I was very disappointed with myself.
- He was disappointed to see she wasn’t at the party.
- I’m disappointed (that) it was sold out.
- She was disappointed not to be chosen.
As you can see, there are many options to choose from. It´s a matter of context which preposition you have to choose.
In your case:
"I am dissapointed in you." and "I am dissapointed with you." are both correct, while "I am dissapointed at you." is incorrect and "I am dissapointed by you." sounds awkward to me. It would be better an expression such as "I am disappointed by (or with) your behavior."