You can remove users from the personal message enabled group site setting, and then staff can still message them. The setting is about who is allowed to initiate a message and you cannot remove admins and moderators from it.
If you want users to be able to initiate messages to admins, you can allow “everyone” to message your admin group in the group configuration. (You can also enable that for other groups like moderators and staff)
Then the user will see a message button at the top of the group page
They can also add a group manually to a new message, but since most parts of the interface related to messages are hidden, you may want to place a link that creates a new message somewhere where they can find it. I would add the link to the sidebar or use a theme component like Custom Header Links (icons) or Custom Top Navigation Links depending on how prominent you need it. The link to create a new message is
{yourForum}/new-message. You can also add the groupname parameter to prefill the recipient ?groupname=adminsUsers can also flag anything as “something else” to message staff.