To prevent a contact from calling you, sending you messages and seeing your state in YouChat you can block it.
This contact will be added in you blocked users list. Which can be found under "Me" tab.
Once a contacted is blocked, you'll not receive any message or call from this user, even if the last one still have the ability to text you.
Anytime you can go and unblock these contacts, from your blocked list. Just press long tap on a user from blocked list, then choose "Unblock User".
In addition to blocking a contact, you can also choose to report them for spam - this notifies us of spammers, so we can stop them in the early stages of their activity.
Block User and Report a spam are not available on Web.