You already have it. You just need to remove one placeholder.
In the email template you have this:
<p>Marketing Opt In: [[+checkboxOptIn:notempty=`[[+checkboxOptIn]]`:else=`No`]][[+checkboxOptIn:default=`No`]]</p>
But you need to remove the first placeholder so it looks like this:
<p>Marketing Opt In: [[+checkboxOptIn:default=`No`]]</p>