SEO spamming is when the optimization method is not accepted by search engines, it can be because of various reasons content which is logically meaningless to a human reader filling a page, which targets to increase the density of a keyword in a page. Meaningless repetition of a word in a paragraph of text, hiding keywords are few spamming in SEO etc. Wikipedia says same about Spamdexing .
If a person manipulate with SEO spamming to increase search engine ranking, it violates the search engine’s Terms of Service.
Spamming Strategies That Can Ruin SEO
Many spammers use this strategy to cheat search engines by “cloaking” that is commonly known as one purpose and serving another, example to show ads. It is a bad strategy and can result in your website being banned by Google.
Plagiarize or duplicate content in website from other pages, this is very known act by spammer and such practices can again lead to bann of website by Google.
Using keywords that are not related
Using keywords on your website that are not related to services or products your website provides is another bad strategy for SEO.
Links from other websites and from your to others, in this website linked from unrelated forums is a wrong practice for SEO Spamming.