Give user rights to delete participants in event records:
  1. Open the security group.
  2. Highlight Records and click Options.
  3. Highlight Participants (Events) in the left frame.
  4. In the right frame, mark View, Add, Edit and Delete for Participants.
  5. Click OK.
  6. Click Save and Close.
  7. Affected user should exit and sign out of The Raiser's Edge for the new setting to take effect.