How to change css style of element inside iframe using css and javascript?

I inserted this calculator: service-online.su/text/calc/calc1-01.php into my website in iframe tag:

<iframe id="myIframe" src="//service-online.su/text/calc/calc1-01.php" scrolling="no" height="370px" width="650px" frameborder="0" marginheight="0" marginwidth="0"></iframe>

But when I click on Compute (“посчитать”) button 2 or more times, inputs above this button duplicate.

I tried to style by id, class, but no result.
How can I fix this bug?
I have no access to the code of service-online.su/text/calc/calc1-01.php.
Please help!