Have at least on checkbox in checkbox group visually(!) checked

I'm looking for a way to have in a group of checkboxes at least on checkbox checked. The only things i could find was methods to validate IF at least one checkbox is checked, but you where always able to check/uncheck the last checkbox - is there a way to make sure, when the user clicks the checkboxes that at least one checkbox has to remain checked (and only if at least two checkboxes are checked one of both can be unchecked) ?

Thanks for your help! Vin