To change who can send you Snaps: Click ghost on the camera screen to access your profile. Then tap the gear icon in the upper-right corner of the screen and scroll down to the "Who Can" section.

• For Snaps: Go to the "Contact Me” option and choose "My friends” – only your friends are then able to send you Snaps. The other option is "Everyone”, but this will allow any stranger to send you Snaps.
• For Stories: tap the "View my Story” option and choose either "My friends” (all of them) or "Custom” (you can select specific friends only). The other option is "Everyone", but this will allow any stranger to see your stories.

To block a contact:

• Tap the username you want to block in your "My Friends” page.
• Tap the gear icon that appears next to their name.
• Select "Block” to prevent Snaps, Chats or them viewing your stories.

To block someone who isn’t in your friend list, swipe on their name on the Chat screen to open a Chat with them. Then tap the button in the top left corner to access their profile, where you can block them.

For more information, visit: https://www.snapchat.com/safety