Edit in JSFiddle

$('#check').click(function(){
     
    if($(this).prop('checked') == false){
         $('#submit').attr("disabled","disabled");   
    }
    else {
        $('#submit').removeAttr('disabled');
    }
});

<input type="checkbox" checked="checked" id="check"><a href="#">利用規約</a>に同意して送信する
<input type="submit" id="submit" value="submit">