I'm assuming that the checkbox is not saved in the database and therefore there are no specific fields in the database to perform the Where statement on.
It could be better to:
1. Create a Checkbox with Control Source Type = "Code Expression" and blank Control Source.
2. In BeforeExecuteUpdate event add code that prevents record update if the checkbox is unchecked:
global $FormName;
if(!$FormName->CheckBox1->Value) {
$FormName->ds->CmdExecution = false;
}
(this will work only starting with CCS 2.3 because CmdExecution was added recently)