I installed Phpbb forum on trap17 hosting with the help of Fantasico thing.Its very great but the problem is that , anyone can post there , i mean even Not registered members can post there and the forum doest even tell him/her that you need to sign in before you post , i want to remove this thing , i want that they should be a registered member and signed in , in order to post any reply or any new topic.I tried to find the option in admin area but i couldnt , can any one tell me how to enable that function.Thanks

You need to set the Forum Permissions to the specific level of security you desire.
Log-on as an Admin user, go to the Admin Control Panel (ACP).

On the left hand side, choose Forum Admin - Permissions and then set each forum according to what you would like them to be. There is a 'simple' method and the Advanced method to select from.

In the Advanced method, there are 10 types of activities which you can set the permissions for : view, read, post, reply, edit, delete,sticky,announcements, voting, and polls.

And you can set them so they allow : all, registered, private, Mods, Admin to have those permissions by each activity.

Sounds like you want the read, post, reply set to registered. And Mods can edit, delete, maybe do stickies and announcements, and registered to vote and start polls.

You may want one Forum set to Mod and another one set to Admin status so the Mods and Admins can chit chat amongst themselves. And at least one Forum set to All so that guests can post and reply in order to promote their involvement in the Forums.

Permissions can also be set according to Users and Groups. It can get complex if you want it to, but there is also the 'simple' method which reduces the complexity. Go to www.phpbb.com or .org to download or read their manual. It isn't that tough to figure out.

You've basically allowed the Guest group to post in your board, that's the reason why without signing in, a guest user may also make a post but identified as unknown user or anonymous.

To fix this, go to your control panel profile settings and configure such that the Guest group is not allowed to post in your desired forums in the board. lhaslip had explained pretty in detail.

Yes , i have solved the problem.I was trying to find that in Management , but was too stupid to look in permissions section, Thanks for your helping answers.

You "manage" the Forum by assigning "permissions".