History of Potal pages visited by users can be controlled by setting value of keymanager.lru.size property in StateManagerService.
This property actually indicates that navigation is affected between portal pages and not portlets.
As per IBM Info Center
"Use this parameter to specify the history expiration limit of portal pages visitedby users. This determines how far backwards users can at least navigate in therecent history of portal pages that they visited. The number that you specifydefines the minimum number of different pages selected by the user after whichthe portal can discard the render parameters of a page. (The decision whether therender parameters of the page are actually discarded depends on the expirationpolicy of the internal cache that stores the render parameters of those pages.) Ifthe user returns to a page after visiting the specified number of other pages and ifthe render parameters of that page have expired, the portal displays that page inits default state."
Note: If this parameter is enabled, number of Render Parameters in a session cansomewhat be controlled.
A typical setting would be keymanager.lru.size = 3.