Once a project is finished or a knowledge-base no longer active, the Moderator can archive it.  You can also choose to change the security settings at that time, if users should no longer be able to access the archived workspace.  Archiving workspaces is a good way to make a workspace out of sight, but still keep the information available to you inside of PBworks.  Deleting a workspace is permanent, and once deleted, a workspace cannot be restored, however, archiving a workspace is reversible by the workspace Moderator.

How to archive a workspace

To archive the workspace, workspace moderators can go to workspace Settings:

Then, click on "Archive..." in the left panel.

Choose whether to change the workspace's security settings and click "Archive This Workspace".

To reverse this action, when you return to the Archive setting you'll be given the option to "Remove from the archive".