Firstly, the permission set must exist. Go to Forum – Permissions > Manage Permission Sets and either edit one to use for this purpose or create one specially.. You basically need a permission set that has every permission in the ‘Moderation’ box UN-checked.
If assigning it to an entire forum group, go to Forum > Manage Groups and Forums, find your forum group and click the ‘Add Permission’ button. Pick your usergroup and pick your permission.
If you have more than one usergroup you want to use the same permission then repeat the process.
If you want to assign it to an individual forum instead of the entire group then you can do that the same way.
It really is worth reading the Codex FAQ’s, getting started guide, pages on permissions, usergroups and the rest. All the information you need is there. And of course if you ever get stuck looking at a panel in the forum admin be sure to click the ‘Help’ popup to find out what it does.
Hope that’s what you wanted to know!