How to prevent negative SEO?
To avoid Negative SEO against your site, regularly practice Positive SEO. It will help to build your reputation in Google and other search engines, and they will not give value to the bad SEO links, etc.
You may find this article very helpful to recover from bad SEO.
Negative SEO (NSEO) refers to several black hat techniques that attempt to penalize the SEO of a rival site. Here are some NSEO techniques and a way to prevent each:
DDoS attack against your site - try to use a web host or CDN that offers to block these kind of attacks (CloudFare for example)
Hacking your site - keep your server up-to-date; for example, the CMS and PHP versions it uses
Copying your content - routinely work on positive SEO techniques to give authority to your site to indicate to Google that it should penalize other sites containing duplicate content, as copied from your own
Hiding bad content or links in your site - protect your site from XSS (Cross-site scripting) attacks...
Links pointing to your site from others like for Viagra, online poker, etc... - routinely work on positive SEO techniques to give authority to your site to indicate to Google not to assign importance to these deleterious links
Many links pointing to your site and their removal in one go - routinely work on positive SEO techniques to give authority to your site so that Google might see that the percentage of these links as compared to your good ones are too small to penalize your site
Social engineering (usurping your identity to ask a webmaster to remove good backlinks you got) - there is nothing you can do except maximizing the number of your good backlinks
Fake profiles on social networks - create social profiles (Google+, Facebook, Twitter, LinkedIn...) before your rival who would be able to speak badly about you, your business and post links to bad sites