Encrypt a message with a one time pad and destroy all copies of the original message. You can literally "decrypt" the ciphertext into anything that matches the same length. The only way to recover the message is to know what the pad was.

I am not sure how well the idea translates down to more practical forms of encryption though, but I can understand the idea in the case of perfect encryption -- you can just make up a different one time pad and decrypt the data to whatever you want it to show, and there is no way to prove if it is really what was originally encrypted.

Edit: I meant to point out that this is in the "what color are my bits?" sense; multiple pieces of data can be "encrypted" to the same result differing only in the "key" (one time pad) used to "decrypt" it. There is no true message in the "encrypted" block without interpreting it in terms of a specific key; how you made the encrypted data tints your bits to the "proper" decryption. It's not a property of the block of data itself.

If the data can be reconstituted meaningfully - i.e. the term "decryption" refers to more than simply making up data, such as the application of a specific one-time-pad or any other method that recovers the original message - then by definition it was not destroyed.

It's a bit ambiguous what it means to erase an abstract entity like "data". Usually, I would accept it to mean that there are no physical copies of the plain text remaining -- e.g. no copies in terms of bits on a hard disk or strokes of ink on paper. If you've overwritten the sole copy of the original plain text with the ciphertext on a hard disk, then I think it's fair to accept that as erasure of the original data.

Philosophically, of course, all strings of data that will ever be written exist in some abstract mathematical sense -- I can write a program that will eventually generate every single combination of bits that can be ever be stored on a given computer (should you care to wait long enough) including novels that don't exist yet (in the conventional sense that a human author hasn't put pen to paper), and the plaintext that was overwritten by the cryptography program.