The most likely reason that you can edit a posted donation is because the validation rules have been shut off. To turn those rules back on:
  1. Navigate to Setup, App Setup, Customize, Opportunities, Validation Rules.
  2. On the Opportunity Validation Rules screen, find and edit each rule.
  3. Check the "Active" box and save.
At the very least, activate the Prevent_Posted_Edits validation rule.