Hi Marti__G,
Welcome to post in our forum.
Regarding your issue, there is no way to make the hang up button end the ongoing presentation automatically, if you want to end the presentation process, you need to manually click the “stop presenting “ button to end it, the behavior is by design.

For your requirement, you could provide feedback in the following link, we will collect all your information and then improve our product.
https://www.skypefeedback.com/
Or you could post this case in Lync SDK forum, there are many experts will help you develop this function.
http://social.msdn.microsoft.com/Forums/en-US/communicatorsdk/threads
Thanks for your understanding.
<p style="margin:0cm 0cm 0pt"><span lang="EN-US" style="color:black; font-family:'Tahoma','sans-serif'; font-size:10pt">Alice Wang <br/> TechNet Community Support</span></p> <br/> Please remember
to <b>mark the replies as answers</b> if they help and <b> If you have feedback for TechNet Subscriber Support, contact <a href="mailto:tnmff@microsoft.com"> tnmff@microsoft.com</a>.