Selection Blocking- Adobe Echosign

I need to send a document in which there is a section showing the signer to choose one option or another, for example,  "yes" or "no". Before I send the document, I click on "initials" and drag it to the Yes line and then to the No line, so that the signer can choose which one they want.

In the past some signers chose both, in error (even after saying "choose one or the other").   How do I make one selection non-chooseable once the other selection is chosen? Is this possible? 

If the signer chooses to initial by "yes", what do I need to do in Echosign to make it so they can't initial by "no" ?

Also what is the phone number for Adobe Echosign? I would also like to speak to a customer service representative and can't find it on the website.

Thank you!