Don't block their IPs, but rather return them subtly wrong content that isn't broken at the first glance. Insert typos, replace important terms, inject nonsense

Don't block their IPs, but rather return them subtly wrong content that isn't broken at the first glance. Insert typos, replace important terms, inject nonsense technobabble, make URLs point to wrong pages, inject off-topic SEO-spammy keywords that search engines will see as the SEO spam they are.