Every client on the Bitcoin network validates incoming blocks, to check that they conform to all the requirements of the Bitcoin protocol. One of the checks is that the block reward (coinbase transaction) is for an appropriate amount. Blocks that fail this are rejected by the client, and conforming miners will not link their own blocks to them, so such a block will not be incorporated into the block chain.

If you tried to make such a block, and then spend the coins you awarded yourself, those transactions would also be rejected as invalid. Other participants on the Bitcoin network would not consider those transactions as valid payment, and would not give you goods or services in exchange for them.

It would be roughly as if you were to walk into a coffee shop and try to pay for coffee using a one-dollar bill, on which you had crossed out "One Dollar" with a crayon, and written "Five". They wouldn't accept it, and they wouldn't give you any coffee for it.