Hello Wendy,
I am not really sure how to stop end user to add their 0 ETC tasks to their timesheets. However, if you want to show an error message to the end user and block the submission of the timesheet when they click on Submit button after entering time against their 0 ETC tasks, that can be achieved using Timesheet Rules available in modern UI.
Steps:
Step 1) Login to modern with admin credentials.
Step 2) Navigate to Administrations -> Functional Settings.
Step 3) Click on Timesheets tile. Here you will see all existing Timesheet rules.
Step 4) Click on New Rule button.
Step 5) Enter appropriate values under Name and Description text boxes.
Step 6) Enter the error message you want to show to the user when he enters time against a ETC 0 task.
Step 7) Under Time Entry Error Conditions section select "Any Entry" radio button.
Step 8) Click on Add Condition button against "Where Time Entry".
Step 9) From the dropdown choose condition as "ETC Is Equal To" 0.
Step 10) Click on Create button to save the rule.
Now every time a user tries to make a time entry for a task which has ETC=0 and click on Submit button, they will get the error message you had set in Step 6 and submission of the timesheet will be blocked.
Hope this helps in your use case.
Regards
-Ashok