1. While in the plugin, under the transaction type, mark the check box or boxes for the transactions you want to delete
  2. Click Delete Selected
  3. On the prompt, click Yes to confirm you wish to delete the selected transaction(s)
NoteDeleting transactions out of the plugin do not refund the transaction.

Additional Resources
What happens when a sign up request is deleted from the NetCommunity plugin in the Raiser's Edge?
What happens if a transaction is deleted