For all events you receive in vene dash, you have the ability to delete them at a later date. Head over to the Events or Postbacks report and you can find the option to reject the event. Please see screenshot below:
Once you select reject, you will have the the option to choose as to why you are rejecting the conversion along with a disclaimer that this action would be irreversible. Once you have selected the required options, the event will be deleted and your reports would be adjusted to reflect the changes.
Note: The deletion of events only decreases the amount in the Revenue and Payout column. The actual Event or Postback counter remains the same. For example if you have 5 Conversions with a revenue of $100 and you reject the conversions, then it will still show 5 Conversions but the Revenue column will be $0.