You can do this manually, by opening the .All Symbols quotepage and deleting the symbols one at a time, by clicking on the ticker symbol in the quotepage and then clicking on the red trash can icon in the quotepage toolbar. You can delete ALL the symbols by clicking on the "Ticker" column heading and then clicking on the red trash can icon on the quotepage toolbar. However, that will remove the symbols from other quotepages as well.
You can create a scan using File > Open > RTL Object. Select Scan at the top of the window.
Change the syntax to replace the quotepage_1 and quotepage_2 names with the names of the quotepages which contain the symbols you want to retain. If there is only one quotepage, change syntax to QP != "quotepage_1" If there are three quotepages, change syntax to QP != "quotepage_1" AND QP != "quotepage_2" AND QP != "quotepage_3"
The results of the scan will be a quotepage that contains all the symbols you want to delete. Click on the red trashcan icon on that quotepage's toolbar to delete these symbols from the database.