It's actually only possible really with template edits... but that's not a big problem.
I assume you're talking about things like post count, custom fields, register date etc. that you can activate in Style Properties > Message Elements?
If so, you need the template message_user_info.
Add the code in red to hide the elements from guests. I've chosen to add the check individually on each element as this will give you the flexibility to pick and choose which you do want to show or don't: