Access mask problem.When you create a forum the initial access mask is set to Admin,therefore if you don't change it all roles have full access.
You may go to Admin > Settings > Forums > edit and then change access mask.
Also if you have changed the access mask itself,then you need to disable all the permission,except "Read" for user guest.The default access mask should be as follow:
Hi thank you for your quick reply. But I changed the all roles only admin having delete permissions. Please find the below roles of my forum
Access Masks
Name Read Post Reply Priority Poll Vote Moderator Edit Delete Upload Download
Admin Access True True True True True True True True True True True Edit | Delete
Member Access True True True False False True False False False False True Edit | Delete
Moderator Access True True True False False True False False False False True Edit | Delete
No Access False False False False False False False False False False False Edit | Delete
Read Only Access True False False False False False False False False False False Edit | Delete
Thank you