For the first error, probably the forum is not configured propery. Check out http://www.sitefinity.com/help/developer-manual/forums-public-controls.html
. We have this problem in the previous releases. Coul you tell us which version you use?
As for the second problem, you could deny modify, delete and change permissions for a given role. Thus one user will not be able to edit other users' posts.( this applies from 3.6 )
the Telerik team