Our recommendation, in general, is to never pay with Bitcoin or another cryptocurrency unless you personally know the recipient. It is impossible to unilaterally reverse transactions once confirmed on the blockchain. Companies who ask you to pay with a cryptocurrency do so to hide their (or your) identity and conduct money transfers which are untraceable for authorities. For more information, read 'Can Cryptocurrency Be Recovered From Scammers?'
If you’ve already paid someone with a cryptocurrency who is no longer responding to your emails and you now believe is a fraud, you can take the following actions:
3. Contact your local law enforcement agency
Again, we do not recommend making payments with cryptocurrency to untrusted sellers. If you like to remain anonymous, you could consider the service Bitrated.
Bitrated uses multisig addresses to activate a third-party arbiter in situations where a buyer is not satisfied with the end result of a transaction. Instead of being stuck with PayPal, Amazon, or one of the handfuls of other online arbiters, you can now find a dispute resolution provider like you would find an Uber driver or an Airbnb rental.
If you’re making an online purchase from an untrusted buyer via Bitcoin, you now have the ability to essentially conduct a chargeback if things don’t go your way. It should also be noted that the money is not held in escrow, so there is no chance for a malicious arbiter to run away with the funds (unless they’re colluding with the seller).