Hello,
How to solve SEO issues related to toxic backlinks and Spam forum Post?

Hi Friends,
You have 2 problems here: linking out and backlinks to your client's site

For backlinks, the answer is fairly straightforward:
Google judges backlinks based on TOTAL number of links

So this means you have to earn / generate lots of clean backlinks to your client's site

The second problem of linking out: delete the forum
Create a blog and link out to high value sites so the HILLTOP update can benefit your site's rankings

If you see your website is still containing back links on high SPAM or toxic sites, you can simply create a list (.txt file) of all toxic sites and disavow them through the Google's disavow tool.

You know you need a disavow links file if:
1.You receive a manual link penalty or a Google link warning in Search Console (formerly Webmaster Tools)
2.You suspect a Penguin algorithmic link penalty, or
3.You believe you might be a victim of negative SEO