how to Disable the Default Appian Accept and Reassign and back button in the tasks

Why?

If you assign a task to multiple people, the somehow need to take ownership.

The accept (and, as far as i know, "back") buttons cannot be disabled under any configuration where a user is able to access a task assigned to more users than just themselves.  As Stefan said, "accept" is absolutely essential since there's no other way for a particular user to take ownership of that task instance.

The "reassign" control can be disabled by setting the Task Reassignment Permissions to "none", though I should caution you that if you view a task as a System Admin user, you'll still see it (though basic users won't).

Adding to what Stefan and Mike said, AFAIK, the accept button will not be visible if you assign this task to a user and not to a group.