Create a validation rule on the Account object is how the app builder can keep accounts with open opportunities from being deleted. According to the Salesforce documentation, "Validation rules verify that data entered by users in records meet the standards you specify before they can save it." A validation rule can check if an account has any open opportunities and display an error message if someone tries to delete it.
Create an Apex trigger on the Account object, remove the delete button on the account layout, and remove the Delete permission from the Sales Rep profile are not valid or sufficient solutions for this requirement.