To do that first ensure your users have a license and permission set to Campaign Calendar as noted here.
Next we'll need to ensure the user profile has their Standard Object Permission for the Campaign Object just set to "Read".
That can be done by going to:
- Setup | Administer | Manage Users | Profiles
- Click Edit next to the appropriate profile
- Scroll down to the Standard Object Permissions section
- Find the Campaign object and uncheck the "Create", "Edit", "Delete" boxes - leave the "Read" box checked
- Click Save.