Hi, I'm working in an ecommerce platform company that has over ten thousands sub-domains created by it's customers, Google is indexing every single sub-domain and it causing a lot of errors and affecting our ranking a lot.

In Search Console we have hundreds of thousands of pages indexed by google 99% of them belongs to sub-domains.

I don't have a lot of experience in SEO, And I don't know how to fix this.

- I can't block each sub domain individually or remove it using the removal tool.

- I can't modify each sub-domain ( robot.txt, .htaccess ) individually.

Help is appreciated.