Is there any way to add content to an email that will prevent the smart reply buttons from showing in a Gmail recipients mailbox? I am sending emails to clients that I do not want the smart reply buttons to show up in their Gmail app as they are incorrect and misleading.
As of October 2018 you can now make a change to your settings that will prevent this feature appearing on emails you receive. That change will apply to emails even if they aren't from Gmail users.
You can think of it a special filter that is applied to incommoding emails. If the application believes the smart_reply filter applies, then you will see the smart reply options when you read the email.
Because it is a special filter on incoming email, you have no ability to modify settings on the recipients Gmail account. If they allowed you that power it would be dangerous.