This simple addon removes the ability for guests to use the contact us form. If a guest tries to access the contact us form, they will get the standard XenForo permission error that they must be logged in.
XenForo groups the guests and unconfirmed users into the same permission groups, thus not allowing admins to set unconfirmed users the ability to use the contact us form and guest not able to use it.
There are no settings for this addon, just install it and forget it.