How to customize a reply for senders, either "accept" or "decline"?

I am not sure if I have understood your question correctly. Do you mean to say that you send to separate email to users based on "accept" or "decline"?

Yes it is possible to achieve your requirement and here are the steps you need to follow:

#1. Add auto-responder email alert in form and edit the template for "Accept" answer.

#2. Add another auto-responder email alert in form and edit the template for "Decline" answer.

#3. Add condition in your form to check user's answer and then send the appropriate auto-responder email alert. You may like to take a look at the following guide which should help you:  https://www.jotform.com/help/167-How-to-Send-Email-Based-on-User-s-Answer 

Hope this helps.

Do get back to us if you have any questions.