As @mikekaganski said, use option buttons.
If only one choice is allowed, then the button should look like
(o) because this is the standard. Otherwise, if it looks like
[x] then people will think it is possible to check both.
If you still decide to use check boxes, then write a macro to uncheck the other box.
Right-click on a check box in Design View and select Control. For the code below, set the Name properties to
chkNo. In the Events tab, use the following macro for the Execute action of both.
Sub MutuallyExclusiveCheckboxes(event As ActionEvent)
model = event.Source.Model
form = model.getParent()
If model.Name = "chkYes" Then
otherName = "chkNo"
otherName = "chkYes"
otherControl = form.getByName(otherName)
otherControl.State = False