The Flexible Sustainer Donation Level doesn't have a setting to disable the One-time gift option, but it can be hidden using CSS. Follow the steps below to add the CSS to your form.
  1. Navigate to Fundraising > select Donation Management
  2. Select the All Donation Forms tab
  3. Search for your donation form
  4. Click Edit
  5. Navigate to step 3. Design Donor Screens
  6. Under Related Actions, click Customize Look and Feel
  7. Under option 2. Custom CSS, add the following CSS:
    • <style type="text/css">
      display: none;
  8. Click Finish