Once you have blocked a user (tap on "More" and "Block user" in the profile above), he or she can no longer send you private messages. In addition, he or she will no longer be able to attend any meetings that you have already joined.
To unblock a user, go to "Profile" ➞ "Blocked users". Tap on the X in the user's picture to unblock the user.
Please do not block other users lightly and be sure you really want to exclude the person from meetings you are already attending.