Log out of the forum, then depending on which browser you use do this:-
Internet Explorer go to Tools/Internet Options
In the Browsing history section click Delete, in the Delete Browsing History box, clear all the tick boxes except Temporary Internet Files and Cookies. Then click Delete
Close then restart Internet Explorer and then
In Firefox (Chrome is similar) go to Tools/CLear Recent History if you can't see the details click Details arrow to get the menu,
tick Cookies and Cache (untick the rest) then in Time Range select everything.
Close then restart Firefox (or Chrome) and then
Go to the Forum Index page and scroll to the bottom and on the right hand side select "Delete all board cookies"
You'll need to log in again afterwards