Slash now offers the ability to lock your cards to certain merchants.
To see your merchant list:
Go to Cards
Click Merchants Allowed
Here you are able to choose between "Any" and "Custom". Any means that your cards are not merchant locked, and can be used at any merchant. Custom means that your cards are merchant locked to the merchants you have selected.
To add merchants to your list:
Go to Cards
Click Merchants Allowed
Select Custom
Search for the merchant you'd like to allow, and click them to add them to your list.
Use the delete button next to a merchant to remove them from your allow list.