How can I forbid team members to edit time manually?

You can disallow editing time manually both for a particular user and all team members at once. 

To forbid a certain user to edit time manually

1. Go to Workspace and click Members. 
2. Click a required member's name to open the Edit Member page. 
3. Turn off the Allow editing time manually option. 
4. Click Save. 

To forbid all team members to edit time manually

1. Go to Workspace and click Settings. 
2. Click Manual Time Editing in the left side menu. 
3. Turn off the Allow editing time manually option. 
4. Click Save.