No it isn't. The 50 bitcoin created by block zero are unspendable (because by intention or error the software never passed block 0 through the validation logic that would enter those coins into required data structures).

But any other payment made to that address/pubkey are perfectly spendable by whomever has the matching private key.

Though that misunderstanding is somewhat common, so it's not impossible that the payee believed that they were sending the coins to an unspendable address.