Stops Words also known as Poison Words are those words filtered by Search Engines,.It can decrease your site's ranking. It may not banned your site but it can penalized your site and drop your SERP

Example of stop words or poison words are those adult words,.
once Search Engine caught your site having these words, it will decrease your site rankings,. Stop words only penalized sites but never banned sites

If we creat forums site then some people post poison words is that also can be punished by SEO Engine?. If yes maybe I need to be careful about this.

Well, I'm not sure if this counts, but I used to run some forums, and spambots would always add "adult" words to there profiles. So I would recommend anyone who runs a forward, to take a look at users that register, but never post. They might have links to bad sites, or some of these poison words in there profiles or signatures. I'm not sure if google picks that up, but its something to think about.

Well, there are some advantages to running a civil "no swearing" forum i guess. I think this means that you will be penalized for putting up "adult ads" at your forums which often contain these kinds of words.

I thought I saw a list of non-adult stop words somewhere, but can't remember where. I guess I should look for it as it would be helpful to have the list in this thread!

Always avoid stop words because these will be hermful for your site and dropping rank