You could do this by hiding (or disabling) the action buttons for the user task. Try this:

• Add the check CSS class to your checkbox field.
• In the CSS section of the CSS and JavaScript page, insert the following rule:

.button-wrap {display:none;}

• In the JavaScript section of the CSS and JavaScript page, insert the following code:

$(document).ready(function() { 
  var $checkbox = $('#Field2-0, #Field2-1, #Field2-2');
  $('.check input').change(function() {
    $(this).is(':checked') ? $(this).siblings('label').css({'text-decoration':'line-through'}) : $(this).siblings('label').css({'text-decoration':''});
    $checkbox.filter(':not(:checked)').length === 0 ? $('.button-wrap').show() : $('.button-wrap').hide();    
  });
});

• Note the $checkbox variable's value. These are the selectors for each input box within your checkbox field. You'll need to change these selectors to match those found on your form.