Remove scroll bar from chat box

Hey,

my website has a chat box through cbox.ws with the following code:

<!-- BEGIN CBOX - www.cbox.ws - v4.3 -->
<div id="cboxdiv" style="position: relative; margin: 0 auto; width: auto; font-size: 0; line-height: 0;">
<div style="position: relative; height: 500px; overflow-y: scroll; -webkit-overflow-scrolling: touch;  border:#6a1014 1px solid;"><iframe src="http://www7.cbox.ws/box/?boxid=337834&boxtag=5rw5kn&sec=main" marginheight="0" marginwidth="0" frameborder="0" width="100%" height="100%" scrolling="yes" allowtransparency="yes" name="cboxmain7-337834" id="cboxmain7-337834"></iframe></div>
<div style="position: relative; height: 75px; border:#6a1014 1px solid; border-top: 0px;"><iframe src="http://www7.cbox.ws/box/?boxid=337834&boxtag=5rw5kn&sec=form" marginheight="0" marginwidth="0" frameborder="0" width="100%" height="100%" allowtransparency="yes" name="cboxform7-337834" id="cboxform7-337834"></iframe></div>
</div>
<!-- END CBOX -->

It works perfect on mobile (the scrolling)… however on desktop it adds another scroll bar to it, which I can’t find in code nor delete. Can anybody help me to fix this? Check out a screenshot here: http://img5.fotos-hochladen.net/uploads/capture2015111it260q9pc3.png

Note that any change should not influence the mobile nor the scrolling on desktop. I just want the unneccessary second scroll bar to be removed.

Thank you in advance.

Sincerely,
Burak