How to Delete Opportunities in Pardot?
As you may know when there's a Salesforce connector set in a Pardot account, Opportunities become a read only object in your Pardot instance, therefore you are unable to create nor delete them from Pardot.
In this case, if you need to create or delete Opportunities, you must do so from salesforce.
Please see this article for your reference (https://help.salesforce.com/articleView?id=pardot_opportunity_syncing.htm&type=0)
If you can not afford deleting the Opportunities in Salesforce, the workaround to delete Opportunities in Pardot would be the following:
1. Unverify your Salesforce connector and delete it.
2. Wait approx 5 minutes and go to Reports > Opportunities
3. Filter the opportunities by Campaign and select the ones you need to delete
4. User the table action to delete the selected opportunities