Yes. Go to the Administration Panel, choose "Groups" from the left menu and then click in the "Permissions" link of the "General" group. Now there is an option named "Read-Only Forums".. Just choose the forums you want to disallow posting for regular users.
Doing that,
you should go to "Users" and then click in "Permissions" for the user you want to allow posting to the forum - considering, of course, that the user is in the same "General" ( or the group you edited before ) group. You'll see that in the "Read-Only Forums" the forums you previously choosed in the Group Permission Control are selected. That's because users inherits permissions from its groups.
So, now, just unselect the forum ( or forums ) you want to allow this user to have posting rights ( or just "Allow All" to allow him to post to all forums ).
That's all.
Rafael
[originally posted on jforum.net by Rafael Steil]