How do I restrict posting on topics to certain users?

How do I restrict posting on topics to certain users?

Administration Panel=>General Tab=>Forum=>Categories and forums=>Choose forum=>Permissions=>tick off what you want=>Save

If there are some members, you may need to create a special group and place these members. Than in the admin panel, choose permissions and tick off this special group from posting in certain categories and/or forum.

When I did this, the result was that all the members could not start new topics. To uncheck one group, I must also unchecks "members". So it seems, I have to add everyone to a group that can post, if I uncheck one group from "Can start new topics", those changes apply to all members who are not in a group.

That is not clear. I would like it so that when someone registers, they can start new threads right away., it is just Mr.X who I want to stop from starting new topics. (Reply is okay.)

Am I doing something wrong, or is this just not possible?

Well can i see a screenshot of the permissions you set?