There isn't a Celltype for Grid that does this yet as far as I know. But you can easily make this happen using a bit of jQuery. You can add the jQuery directly into the custom field's instructions or use CP CSS & JS.
This will ensure that only one checkbox remains checked across a particular column in your Grid field (tested with EE 2.7.1):
<script type="text/javascript">
$(function() {
var checkboxes = $('#field_id_28 td[data-column-id=2] input');
$('#field_id_28 td[data-column-id=2] input').change(function(event) {
checkboxes.prop('checked', false);
$(this).prop('checked', true);
});
});
</script>
Just be sure to set field_id-x
and data-column-id-x
in the selector to match your field and column.

Important note: If you are entering this jQuery snippet in a custom field's Field Instructions make sure it does not use space characters for the indentation. If you copy and paste directly from here it will and they will be encoded to
which will cause a JavaScript error.