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Look in the preferences for the modules. You can set it up there. Also, after you make the change, it will only take effect for users that register after the change. Everyone else will retain thier default settings, however you can go into thier profile and make the change manually, or make the change directly in the databaes if you have a lot of users.
To prevent users from being able to change thier view style, set the View Mode of the Threads to "None". You can also find this in the forum preferences. You'll need to be runnign at least newbb2.x in order to turn the view mode off.
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Oh, I don't think the latest version of newbb has been posted in the module repository yet, so you'll need to get it form the Dev Forge.
http://dev.xoops.org/modules/xfmod/project/?group_id=1001
- Ryan