The only one that's correct is, "I won't let you BE disappointed.
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We use disappointed with the conjugate 'to be.'
I am disappointed.
You were disappointed.
He/she has been disappointed.
They will be disappointed.
Hence, when you want to use it in the future tense still it will be with the conjugate 'to be.'
..and also when it is negative it will still be with the conjugate 'to be.'
I/You/He/She/We/They won't be