In the same dialog box (field/control | setup...) you should see the name of the value list that results in check boxes for your field.
Open Manage | value lists... and find this value list.
Select this list and click edit to change the values that appear as check boxes in your field.
Perfect! I'm all set. Thanks!
One word of caution. Editing the value list does not change the information stored in the field.
If John Smith was the previous name in the value list, John Smith will remain in each field after you remove this name, but it will be invisible to you or other users. You may want temporarily add the name back into your value list, find all records with this name, update them and then delete the name from the value list to eliminate this hidden data.