Once you have engaged a client/prospect you do have the ability to enable/disable the periodic email reminders on that specific engagement prior to sending/editing the engagement.
However, if you wish to disable/enable the reminders after the engagement has been sent, click on Engagements > All Engagements under Form Engagements> Click on your clients engagement
Once the details panel is open on the right hand side you can then click on the customize button.
You can then click on the enable radio button to enable any of the available email messages for this engagement.
Managing the reminders prior to sending an engagement 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 email messages button.
To enable click on the enable button then click on the Periodic Reminder tab.
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 under Form 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.