How can we restrict permissions for users to Delete Project?

1. If you want to restrict a user group from deleting only a particular project then, navigate to 
Administration>Project user group>Edit a Project Group>Select the project and the user group> Under Manage Rights you shall find list of rights.
Unselect the 'Allow to delete project' right. Hit next and save the results.

2. Now if you want to restrict this right across all projects i.e. at global level, then navigate to Global right management>select the user group>uncheck the 'Allow to delete project' right. Hit Apply

If you check the check box 'Allow the specified permission in all the existing project for the selected user group', then that user group will not be able to delete the existing projects as well. However, this(for existing projects) may not apply if a user's admin level is 'Venio admin', user group 'Site admin' and client is 'Internal'