How do I restrict mail sending option to a Support Rep?
You can setup a new Profile by restricting mail sending option and associate the Profile to Support Reps.
To create Profile, Goto Setup > Admin Settings > Profiles >Add new profile >and un-check the Mail Send option and Save the Profile.
Goto Setup > Admin Settings >Support reps >edit the support rep and associate the Profile.
Note: You can see Profile link only if you add the Second Rep for your organization.
For more, refer Profiles and Support Reps.