Filter out all crossovers on Ao3.
Ao3 doesn't have a built-in way to filter out crossovers (works that involve two or more fandoms) from your searches. This script implements that: while it's activated, any works tagged with more than one fandom tag will be hidden from your results.
The method it uses to work out what's a crossover (simply: is it tagged with more than one fandom tag?) leads to some amount of false positives. For example, a fic tagged with Marvel Cinematic Universe and Captain America: The Winter Soldier would be hidden, despite it not really being a crossover. However, you're also guaranteed to not find any (properly tagged) crossovers, so if that's what you need then it'll do the job.
If you need the opposite of this, I've also uploaded a script which filters out anything that isn't a crossover. Don't run them both at once, though, or you will not be able to view any works.