"rel=nofollow" is the only somewhat standard one. I have never heard of "rel=ugc" and I wonder if google made it up, does that work on bing and yandex?

I mostly use "rel=nofollow noopener noreferrer" which should cover most "use-cases" of spammers.

In particular:

> rel="ugc"

> We recommend marking user-generated content (UGC) links, such as comments and forum posts, with the ugc value.

That won't stop the spam appearing, and the spam accounts to generate it, it'll just remove (most of) the benefit to the spammer.

It takes less faf for them to just post anyway than to check if you use nofollow/other, and they may still get a small side benefit if a blog-spam scraper or such takes your content and reposts it without the extra directives.