Once you have engaged a client/prospect you do have the ability to enable/disable the periodic email reminders on that specific engagement.
Click on Engagements > In Progress (if it's not started yet you won't find it here but rather on the All Engagements subsection) and as long as you see the progress bar (can be at 0%) click on the engagement to open the details panel.
Once the details panel is open you can then click on the enable/disable button for the periodic email reminders.
Managing the reminders prior to sending a form to a client/prospect
When you're engaging a client/prospect, you have the ability to enable/disable the periodic reminder emails prior to the engagement being sent. Once you've selected the template you'll be able to enable/disable the reminders
To customize the reminder interval/email click on the Customize notifications button.
From there you can customize the subject link, the body of the message and the interval.
How to set up the email reminder on the template on a global level.
You can set up the reminder email on the template at a global level by going to Templates > My Templates > Click on your template > click on Customize > then, while in the template editor click on the envelope icon from the top navigation. Here is where you'll be able to not only set up your periodic email reminder for this template so that you do not have to customize it each time this template is used but also the new engagement email and the Thank You email as well.