Sendy links in Google Index even though they are blocked through robots.txt

I'm sorry if this isn't the right subreddit (point me in another if not) for this question, but I'd really appreciate any help I can get.
I have a client who uses Sendy to send emails to subscribers and it works fine. The problem is that Google indexes Sendy link and shows them in Google Search. I tried blocking Google by putting a rule in robots.txt but Google ignored it. Then I tried to disable access by putting restrictions in .htaccess, another failure. It even messed up email tracking which is important to my client. So is there any solution to this problem? Can I put some sort of 404 redirect to this folder in .htaccess and don't mess up everything?
Is Search Console's temporary removal option any good for a temporary solution to this problem?
Thanks