Sometimes you want to remove contacts. That is possible in most cases. The only ones you cannot remove are co-workers.
In all other cases, simply open the chat with the person you wish to remove, and click on 'more' at the top of the chat screen.
Here you will see the options to remove a contact, or remove and block a contact.
Choose the first option if you only want to remove the contact. This will remove the contact from your list but the contact will still be able to send you messages and reappear in your contact list. If you want to prevent that from happening, use remove and block contact.
The contact will not be able to send you messages again. You can unblock them by simply inviting them again.