the main idea is to try to disable multiple checkboxes using multiple ID'S for example using documentGetElementById
Each id belongs to a checkbox
function main(){
var a = document.getElementById("actual").value;
var b = document.getElementById("destination").value;
if (a == "Jamaica" && b == "Paris"){
document.getElementById("A", "B", "C", "D").disabled = true; // occupied seats
}
}