Bitcoin addresses do not get "reserved" when they are generated, it is just astronomically unlikely to generate one that has been used before. In theory, you could keep generating new addresses until you hit one that has coins in it, but as SgtSpike said this would be much more difficult than finding valid blocks.
Huh... So, technically, it is possible to get into any ones wallet like that? That's fun)
It's also technically possible for me to call your bank, guess your account number, guess your social security or other credentials, guess the right answer(s) to the security question(s) they ask, etc.
Guessing your keypair/address is no different. It can be done, it's just so astronomically unlikely that you'll guess correctly that even at GPU speeds you'll be guessing for longer than the coins are likely to stay in that wallet (read: longer than I, and several generations of my descendants am/are likely to live).