Choose History > Modify to remove items from the History > Items menu. A dialog appears that contains all of the items in the current History menu. Select the items you wish to retain , and click OK . Any items which were not selected in the dialog are removed from the history list.
Note: another way to keep track of items that you're interested in (such as appear in the history lists of various tools) is to place them on the Object Clipboard. See The Object Clipboard for more details.
LispWorks IDE User Guide (Macintosh version) - 25 Nov 2011