If you'll not place the opt out link even in the contact campaign, then users might report it as spam.
In emailing campaigns general practice is to provide the unsubscribe link. Where people can click and unsubscribe their email from your mailing list.
In contact form campaigns, opt out works like this:-
1. Your redirect domain is abc. xyz (Note: never ever use your money site domain directly in contact form campaigns)
2. Opt out/ Unsubscribe link would look like this: abc. xyz/unsub?=
http://targetdomain.com
3. When the user will click on the opt out link, it'll redirect the user to the unsubscribe page with his domain already filled in the text field.
4. User will click on the Submit and it'll add the domain in the unsubscribe.txt on your hosting server.
Just add the unsubscribe.txt link in your GSA website contact blacklist settings. So, it'll ignore the domain from sending the messages further.
Note: It needs a bit of coding skills to code these few lines of unsub php script.
