Negative SEO related problem

What is Negative SEO to you? What kind of Negative SEO is your competitor doing for you? How you found your competitor is doing Negative SEO for your site?

Because Negative SEO doesn't exist anymore as after penguin update Google is not concerned over Negative SEO anymore. Google now devalues web spam and doesn't demote it and now handling all of this by its own so If you don't want any of SEO thing to be in Google Database which you counting SPAM, simply disavow them.

Doing Negative & SPAM linking to your competitor website will not lead to anything, even it can be beneficial for them.

First tell me how you are sure that they are planning and doing it?

First of all, if competitors want to make a negative impact on your SEO, they would start with building low quality links or redirects to your site. You need to always control the profile of your backlinks - Ahrefs can help you out. (or google for more tools to track backlinks)
Always double check for duplicate content, and make sure your content is unique and can be found only on your website (even if you share it elsewhere, make sure to perform couple of changes)
Make sure your website is protected form malware, and actually loads fast. And since social media can also affect your SEO ranking, you should be aware of all the mentions your website or brand has in social media.

In terms of wether or not your should do the same towards your competitors - I'd say rather not. Instead of chasing them, focus on the value you provide to your visitors and on your points of difference. If you provide quality, it will be obvious who's winning the race. Besides, don't hesitate to speak to them about shady methods they use against you. For instance, if you google your website's name, and you see that the first website appearing in search is your competitor, and he's stealing your traffic, misleading the users, you can speak to them, and agree to play fare.