In SharePoint 2007 web sites, web parts can be removed from a web part page using two methods:
Closing the web part (by clicking the "X" button)
Deleting the web part
While the second method actually deletes the web part from the web part page, the first method only hides the web part, maintaining active the association between the web part and the page. In order to restore the web part into the page, the following steps must be performed:
Site Actions à Edit Page
Select "Add Web Part" option
In the Add Web Part page, click "Advanced web part gallery and options"
Select the "Closed Web Parts" group
Drag the closed web part(s) into the desired web part zone
If you want to delete all the closed web parts from the web part page, the best way is to use the Web Parts Maintenance page. The fastest way to access it, is to append "?contents=1" to the page's URL, for example, http://mySharePointSite/Pages/Page.aspx?contents=1 (http://mysharepointsite/_layouts/spcontnt.aspx?&url=%2fPages%2fHome.aspx). In this page, all the page's web parts are listed with their current state (including the visibility on the page). To delete the closed web parts, just select all the hidden web parts (the ones that have the value "No" in the "Visible on the web page" column) and click the "Delete" button on the top of the page.