I saw this answer containing a dead link to a website with lots of ads that does not redirect (I used a redirect tracker. So how can we alert the mods to delete it?
Flag as spam or for moderator attention.
With enough spam flags a post is deleted without a moderator intervention.
You can find more info on this Overflow Blog, aptly advertised on the right of the screen as I am writing this answer.
Another option is to edit the post, and either remove the spam links completely or replace with a better link.
Eg, links to ebay expire after a while, so grab a useful image and insert that instead.
If it is a dead link, then you can check if the page was archived at the wayback machine, or find another URL that serves the same purpose.