Download the Bitcoin Core wallet: signing addresses with that wallet is very easy.
For any doubt or information feel free to ask.
Best regards.
But we need a signature from the address that user has previously used (and posted in a PM).
Hence only the 131pB address comes to mind.
However, if the user is unabled to export the private key from blockchain.info (or get to it in any other way),
downloading core won't be that much a help with this task.
Best regards.