You can hide the email communication opt-in checkbox by editing the Communication Opt-in data element in the donation form.
  1. Select Fundraising > Donation Management
  2. Select All Donation Forms
  3. Find your donation form and click Edit
  4. Click 3. Design Donor Screens
  5. Click Edit next to Donation Form
  6. In the box on the right containing selected data elements, find and click Communication Opt-in
  7. Click Save Order and Edit Selected
  8. Under Hide communication opt-in checkbox, mark "Yes, hide the communication opt-in checkbox. New constituents will be automatically opted in to communication."