In Salesforce we can stop the approval process email from user setup. Follow the below steps to stop the approval emails to a particular user.
Step 1 : Go to Setup
Step 2 : Enter User in Quick Find Box
Step 3 : Click User which you want to restrict the approval email
Step 4 : Click Edit
Step 5 : Set the “Receive Approval Request Emails” to Never
Step 6: Save
In this situation the user won’t get the approval process email even though he is assigned in approval process. You can use this setup to restrict the sandbox approval email if you want.