You could do something similar to the Reject UI Action which requires a comment when rejecting an approval:

var comments = false;
if(current.comments.getJournalEntry( 1) != undefined) {
	var lastComment = current.comments.getJournalEntry( 1);
	var index = (current.comments.getJournalEntry( 1)+'').indexOf(gs.getUserDisplayName());
	if (index > -1 && index <= 22)
		comments = true;
}
if(!JSUtil.nil(current.comments) || comments){
    current.state = 7;
    current.update();
}else{
	gs.addErrorMessage(gs.getMessage("Please fill in the Comments field before closing"));
	current.setAbortAction(true);
}