To void a same-day transaction in Authorize.net:
1. Login here:
2. In the left-side navigation, under the header "Search", click on "Unsettled Transactions".
3. Find the transaction. Click on it.
4. There's a Void button at the top. Click that.
Note: you have to do this by approximately 5 pm on the day the purchase was made, otherwise the transaction will 'settle' at whatever time your settlement is (often about 6 pm).
If a purchase is made after the time at which your Authorize.net usually settles, you have until approximately 5 pm the next day to void it.