I have been scammed by a person and I want to dispute a bitcoin transaction.
Bitcoin transactions cannot be reversed unless some kind of escrow is implemented. Take this as a hard learned lesson. Sorry for your loss.
Bitcoin is designed to put you in control of your own money. While this allows you to "be your own bank", it unfortunately also leaves you without the safety net present in payment services provided by the banking network. Bitcoin payments are pushed out by the paying party instead of requested by the payment receiver.
By design, there is no central referee in the system that can perform chargebacks or mediate disputes.
Essentially, you have similar options as if you had paid your business partner in cash: You can either take it up with your business partner or report them to your local authorities for defrauding you.
If you've made the purchase through a marketplace, the marketplace operator might offer a conflict resolution procedure.
File a police report and let the law do it's magic.