If your version of PHP-Nuke uses the standard phpbbtonuke forums, Last Visit™ can be set to either delete inactive member's forum posts and messages or it can convert these posts to an alternate username, such as Former Member. If you do not have the standard forums, this feature will not be available to you.

In the example below, Last Visit™ is set to retain all member posts and will convert them to Anonymous user.

It is important the the user ID be accurate or the posts will be lost. I recommend creating a username specifically for this purpose.
Also, if you do not allow guests to post on your forums, do not set the user ID to 1 or the posts will simply be deleted by the system anyway.