Well what I'm suggesting isn't about blocking the bots, it's about removing the incentive. So in this case, I think the more obvious it is the better. I would want them to realize as soon as possible that they are 100% wasting their time.
If anything, it might be best to return a page that explicitly states "Sorry, this search engine no longer supports SEO footprint search queries."
On the other hand, making content difficult to parse is easy to do and a very strong weapon. Make them waste dev time... It is much easier to make variants of HTML than it is to parse it. You can even automate it to some degree.
If anything, it might be best to return a page that explicitly states "Sorry, this search engine no longer supports SEO footprint search queries."
*edit for typo & wording