To block someone on Snapchat without them knowing, open the app and swipe right to go to the chat screen. Find the person you want to block, then press and hold their name. From the menu that pops up, select Manage Friendship, then Block, and confirm it.
If the person isn’t on your friends list, just use the search bar to find their username, tap their profile, tap the three dots in the upper corner, and choose Block.
Snapchat doesn’t alert people when they’re blocked they simply won’t see your stories, messages, or profile anymore.