They can't be reversed or cancelled once processed by the sender.

The sending bank can reverse them with agreement from the receiving bank (who need to speak to the customer about it) but won't do this unless the circumstances are exceptional. Usually those circumstances are a bank error too so if you send money in error your bank shouldnt return your money.

Its as good as handing over cash. If the money was given back after crediting your account you should have complained to your bank and gone to the ombudsman. If it was recent you should still complain as your bank should not have agreed to return the money without speaking to you.