Any thoughts on how we can prevent this situation?
1) Fraud is higher with CNP (Card Not Present) transactions, not the least because it's easiest to get away with. If the purchaser says they never received it and files with the card company, the card company takes their side unless you have a signed slip - which CNP transactions never have. So for that particular type of fraud, there's little you can do, except evaluate what it costs your business and adjust your prices to compensate.
2) Your processor (Cielo in this case, apparently) should be performing various anti-fraud steps to determine if the transaction is likely to go through or not. Some processors work harder at this then others, so you may want to look into your alternatives for payment processing. A processor that puts more effort in at that layer might cost more, but it might also be worth it. (Disclaimer: I work for a processor, but know nothing at all about Cielo, and don't know if they're likely to be doing a good job for you or not - go forth and research). These are steps that the processor can take but you really can't, because you don't have access to the same amount of information they build in the aggregate and the level of information they get back from the credit card acquirers. So it's not a build-it-yourself type of thing.