I don't think you can disable the Sync for the guest users, but perhaps other options will work. Here is what another Greg is suggesting:
Option 1: Disable sync at the library level
- Navigate to the library where you want to disable sync, then click Gear Icon > Library Settings
- Click on Advanced Settings
- Scroll down to the middle of the page, and under Offline Client Availability, select No (default is Yes)
- Click OK at the bottom
- You will now notice that the library lacks a Sync Button
Option 2: Disable sync at the Site level
- Navigate to the site where you want to disable synchronization. Click Gear Icon > Site -Information
- Click View all site settings. If on the classic wiki page, you can click Gear Icon > Site Settings instead of these two steps.
- Click on Search and offline availability under Search
- Under Offline Client Availability select No (default is Yes)
- Click OK at the bottom of the page
How to disable sync of user’s OneDrive
- App Launcher > Admin
- Navigate to OneDrive Admin Centre
- Click Sync, then check Allow syncing only joined to specific domains checkbox then
- Specify the list of domains you want to allow to sync.
- click Save.