Yes, it is possible to lock a drop down cell so that only drop down entries can be selected. Here are the steps to do it:

1. Select the cell or cells that you want to lock.
2. Right-click on the selected cell(s) and choose "Format Cells" from the menu.
3. In the "Format Cells" dialog box, click on the "Protection" tab.
4. Check the box next to "Locked" to lock the cell(s).
5. Click on the "OK" button to close the dialog box.

Now, the cell(s) are locked, but you still need to protect the sheet to ensure that users cannot edit them. Here's how to do that:

1. Click on the "Review" tab in the ribbon.
2. Click on the "Protect Sheet" button in the "Changes" group.
3. In the "Protect Sheet" dialog box, select the options that you want to apply to the sheet. For example, you can check the box next to "Select locked cells" to allow users to select the cells, but not edit them.
4. Enter a password if you want to prevent users from unprotecting the sheet without the password.
5. Click on the "OK" button to protect the sheet.

Now, the drop down cells are locked and users can only select entries from the drop down list. They cannot enter anything else into the cell. If they try to edit the cell, they will receive an error message saying that the cell is protected.