The Bitcoin created by the genesis block cannot be spent, ever.
I thought the coins could be spent if anyone has the private key of the receving address for the reward.
Nope, a quirk in how Bitcoin as a whole works means that the 50 BTC mined from the Genesis block cannot be spent at all.
Even if you have the private key of the receiving address, you cannot send it anywhere.
EDIT: Theymos explaining it in better detail:
https://www.reddit.com/r/Bitcoin/comments/1nc13r/the_first_50btc_block_reward_cant_be_spend_why/cchdhrm