It is not possible to undo Bitcoin transactions. As a recipient, you can (voluntarily) decide to send the money back, but you cannot revert the transaction. This is one of the reasons why it's recommended to only transfer Bitcoins to organisations and individuals you know and trust. A Bitcoin address is both long and complicated, which is why we advise Bitcoin users always to copy-paste this address when making a transaction. The chances of you making a mistake when manually re-typing the address are quite significant. Fortunately, more and more bitcoin wallets start to offer features recognising typos. This blocks the system from completing a transaction to an invalid address. We expect more of these kinds of security-oriented features and services to appear in the near future. Until then, it is advisable always to double-check the validity of a Bitcoin address before you use it to receive Bitcoins.