Once a transaction is confirmed on the blockchain, it's irreversible. This is a fundamental property of decentralized blockchains and not something Koala Wallet or any single entity can control.
When you receive a transaction into your address, you can rest easy knowing that no one but you can send the assets out. But the reverse situation is also true. When you send assets out, only the owner of the destination address could ever send them back.
If you know the owner of the destination address, you should contact them and explain the mistake. It may be possible for them to retrieve the assets or send them back to you. If you made a mistake and entered an address that has no owner, no one can send the assets back, and they will be irretrievably lost.
When sending, please always take a few seconds to double check that the asset being sent, the amount you are sending, chain number and destination address are correct.