set limit in selection for sumo dropdown

You can use sumo methods unSelectAll and selectItem and the triggerChangeCombined option on the plugin init.

Ref: http://hemantnegi.github.io/jquery.sumoselect/

In the change event if the limit is raised you can deselect all and set the last valid selection by the index of each element.

Code:

$('#island').SumoSelect({ triggerChangeCombined: true, placeholder: "TestPlaceholder" });

var last_valid_selection = null;
$('#island').change(function (event) {
    if ($(this).val().length > 2) {
        alert('You can only choose 2!');
        var $this = $(this);
        $this[0].sumo.unSelectAll();
        $.each(last_valid_selection, function (i, e) {
            $this[0].sumo.selectItem($this.find('option[value="' + e + '"]').index());
        });
    } else {
        last_valid_selection = $(this).val();
    }
});

Demo: http://jsfiddle.net/IrvinDominin/80xLm01g/