These NSR are just not spendable. It’s enforced by the protocol: nobody can sell them. They are burned.
If he does all the operation under the same username and nobody else claims the burning it’s not really needed unless there’s a possibility his forum account may have been compromised.
He can prove it by signing a specific message (for example something containing his username and his NSR custodian address) with the private key of one of the inputs of the burning transaction(s).
Shareholders are responsible for not allowing too much burning. When new proposals appear they can adjust their vote if that changed their mind.