After some discussion with Andre we decided:

1. We keep both buttons always enabled. Reasoning: We do not want to disallow a valid operation just because our heuristic says that attempting a decryption makes no sense.
2. Instead of the Encrypt button we switch the Decrypt button to Import if we detect a key block. This way the users can encrypt key blocks (which does make sense; in particular, for protecting exported secret keys), but attempting to decrypt a key block will always fail.

I have implemented this a bit differently in particular with usability (e.g. discoverability of the import possibility) and accessibility in mind:

• Add a separate Import button instead of re-using the Sign/Encrypt button.

For one, this allows the user to encrypt a public key block. Moreover,
buttons that magically change their meaning are bad for accessibility.

• Update the three crypto operation buttons in one place.
• Disable the Verify/Decrypt button if the notepad is empty.

works as described.

But what I do not like is that you get a general error if clicking "decrypt" with a public key block in the notepad. I'll make a new ticket for that