A cryptocurrency transaction that has been confirmed and posted to the blockchain cannot be cancelled or reversed.
A cryptocurrency transaction that has not yet been confirmed but has been submitted and is currently 'pending' can usually be cancelled, but not always. It is important to act very quickly if you want to cancel a pending transaction, because once it's confirmed it cannot be cancelled.
In order to cancel the transaction, it's first important to understand whether you have sent the transaction from a self-custodial wallet of yours, or an exchange account, and furthermore important to understand what cryptocurrency was being sent, and the wallet software you were using (if using a self-custodial wallet).
If Using a Wallet:
There are two primary ways a pending Bitcoin transaction can be cancelled. A transaction is pending when it has been submitted, but is waiting to be included in a block on the blockchain. The two ways are Replace-by-Fee (RBF) and Double Spending. RBF is the ideal option, which involves the user submitting a replacement transaction to effectively overwrite the pending one.
However, RBF is only an option if the RBF protocol was enabled with the pending transaction was sent. Usually RBF is opt-in, by some wallet software enables RBF by default. You'll also need a wallet capable of submitting the RBF transaction. If RBF was not turned on in the initial transaction, RBF is not an option.
Attempting a double-spend transaction is the less ideal option, and undoubtedly will prove to more difficult to achieve. This is because most wallet software implements safeguards that prevents users from double-spending Bitcoin in their wallet.
The process for cancelling Ethereum transactions, as well as other EVM-compatible cryptocurrencies like Polygon, Binance chain, AVAX C-chain, Arbitrum, and Optimism is entirely different.
The easiest way to cancel a pending Ethereum transaction is to create a new transaction that uses the same nonce as the pending Ethereum transaction you want to overwrite. The new transaction must have a notably higher gas fee so as to ensure it gets confirmed before the pending transaction. The new transaction can be a transaction for $0 worth of crypto, and you can even send it to yourself.
If Using an Exchange:
It is important to understand that a pending cryptocurrency withdrawal transaction from an exchange may not necessarily be pending on the blockchain. The 'pending' status you may see in your exchange account is in reference to the exchange's internal systems. Whether or not a pending transaction can be cancelled, regardless of currency, is dependent on the exchange and whether they allow their customers to cancel pending withdrawal transactions.
If an exchange allows users to cancel pending withdrawal transaction, the ability to cancel can likely be found in one of two places:
1. The 'withdrawal' history on your account -- often pending withdrawals will show up here too, along with a 'cancel' button.
2. An email from the exchange sometimes allows gives users the option to cancel pending transactions. Note that if the exchange requires the users to pass an email confirmation for each withdrawal then the 'pending' transaction may never be posted to the blockchain, and would effectively be cancelled.
