How can we save our client's websites from penalization or to be banned?
You have to avoid below techniques while optimizing a site:
1. Over optimization
2. Black Hat SEO (Unethical SEO)
3. Low quality & Irrelevant backlinks
4. Buying backlinks
5. Disavow all low quality and bad links
Don't use duplicate content or illegal means of on page optimization
Regarding off page avoid links from bad neighbors
Always search links from quality relevant websites