A spammy website may keep backlinking to your site. If, for some reason, the website hasn't shown up in Monitor Backlinks yet, you can add it manually to disavow.
First, go to the Your Links dashboard and click on the + sign to add new backlinks manually:
After you add the new backlink(s), make sure you disavow the domain (not just the URL) so that you can completely remove all backlinks and future backlinks from that domain in one fell swoop!
Do that by:
- Clicking on the gear icon of the backlink
- Clicking "Disavow," then "Disavow domain:"