If you don't want certain users to be able to contact you on Session, you can block them. Here's how:
iOS
Open the conversation with the user you want to block. Tap on their profile picture in the conversation and toggle on "Block this user".
Android
From your list of chats, long press on the profile picture of the user you want to block. Select "Block" (🚫) from the menu and confirm.
You can also open the conversation with the user, tap the three dots (⋮) in the top right corner and select "Block".
Desktop
From your list of chats, right-click on the profile picture of the user you want to block. Select "Block" from the menu.
To learn more about blocking, check out this video: