User level restrictions

Is there any way I can restrict my all clients on adding mail contacts through HC10 panel?

You can configure restriction on adding of Exchange Mail Contaccts for your customers by following below mentioned steps :
 

1. Login as Global Admin user in HC10.
2. Navigate to menu : Configurations >> Panel Conf. >> Manage Role Permissions.
3. Here specify Select User Role as Reseller/Client.
4. Now under Exchange >> Mail Contacts link select check box against "Do not allow to perform 'Add' action".

 
This way, now your customers would not be able to add Mail Contacts via HC10 panel.