Can I use /robots.txt or meta tags to remove offensive content on some other site from a search engine?

No, because those tools can only be used by the person controlling the content on that site.
You will have to contact the site and ask them to remove the offensive content, and ask them to take steps to remove it from the search engine too. That usually involves using /robots.txt, and then using the search engine's tools to request the content to be removed. For example, see: How can I prevent content from being indexed or remove content from Google's index.
If that fails, you can try contacting the search engine administrators directly to ask for help, but they are likely to only remove content if it is a legal matter. For example, see: How can I inform Google about a legal matter?