From main menu, go to ‘Contacts’ tab and select the contact you wish to remove/delete. You can use bulk action, and select a few or all contacts at the same time.
Once contacts are selected, click on ‘Actions’ button, and choose delete.
In order to complete the removal process, you will need to enter the name of your organization.
Click on ‘Delete’, and contact(s) will be removed from system.
*Once deleted contact cannot be recovered.
For deleting contact from deal, check this article.