All members, facilitators, and Member Hosts can block members from contacting them via chat.
Blocked members won't receive notifications. They'll see a message when they try to message you, explaining they can't.
To block a member, click on their profile and scroll down to Block this member. You can also block a member directly after attending a session together.
Blocked members appear on your blocked list, found in Your Profile > Blocked members, where you can manage them.
Related: flagging & reporting someone