All the users you have blocked are listed in the "Black List" section in your profile Settings. You can unlock users in this section by selecting the "Unblock" option.
To block a user, start chatting with him/her. To the right of his/her name on top there is an icon shaped like three circles. Click on it. Then press "Unblock" and confirm your action.
Need more help? Email us at firstname.lastname@example.org.