I've got version 3.4.03 and was playing with a read-only forum. The soloution I found was to lock the forum. This works fine untill the moderator is trying to edit a topic. (Error message: Page can't be displayed when klicking "Post Changes"). The moderator can edit the replys without any problem.
For both the admin and the moderator its displays the same url. The only difference is that the admin is comming back to the forum and the moderator get stucked with a page not found.