You can do a variety of things to lower the potential reach -
1. Narrow the targeting down using a behavior of some sort, say purchase behaviors like Engaged Shoppers, Credit Card Shoppers, Online Shopping. Facebook gathers data from 3rd party sites and finds people who buy stuff online or through Facebook itself and uses this data to deliver your ads to will be more likely to purchase something from your site. Also, turn OFF interest expansion.
See how I narrowed down a 19M audience to 6.3M using a behavior? Now the ad is gonna deliver to people who've bought something online recently through Facebook or an affiliated site. If you've already done this and it's still large, let me know. Sometimes running an ad with a broad audience isn't quite bad either. One of my most profitable adset ran to a 58M audience and did extremely well.
2. Separate the genders - If you're running this adset to both genders simultaneously, edit it and change it to a specific gender. Duplicate the adset after that and select the other gender. Don't run an ad to both genders at once, it's harder to read data later on.
3. Filter out by states - I assume you're running this ad to the United States, if so then select about 10-15 states only and that should significantly lower your reach.
I hope that helps.
