[xiphias2]

Have you thought of the option of disabling links? That would make SEO spam impossible

[pitay]

Is just adding the attribute rel="nofollow ugc" to any links in submitted content may be good enough. This tells search engines to not index, or tag them as suspicious, allowing them them to identify SEO spam more easily. [1]

Having both options would be great.

[1] https://support.google.com/webmasters/answer/96569