How can I remove a spam page from my website index?

You’re not understanding the issue. “Vendors” is not a collection on our store (and I would expect this is the case for many of the other stores in this thread), so /collections/vendors/ shouldn’t exist. A 404 is the expected response. On top of that, “q” is getting passed into the collection title ( {{collection.title}} in the template outputs the query parameter) for some reason. So, the spammers simply pass anything they want into “q” at /collection/vendors/ for their exploit.