If the Service Account is in the Admin group, then he will be granted explicit deny on send as / receive as. You cannot also make a change and expect it to be instantaneous, it requires replication which could take some time, or you can force it.
Also you can do the following to turn on the Security Page, to make your life easier
The Exchange 2003 and 2000 System Manager
The Situation:
You have navigated to the Exchange 2003 System manager and now you want to change permissions for objects in your Exchange Organization.
The problem:
Exchange 2003 System Manager does not display the Security Tab, so you cannot modify the permissions. The default position is that when you check any of the Exchange Organization objects, there is no Security Tab.
The solution - Security TabShowSecurityPage Registry setting DWORD
To display the permissions tab, make this change to the registry.
1. Here is the correct path to navigate: HKEY_Current_User\Software\ Microsoft\Exchange\ExAdmin
2. Once you reach the above section of the registry you need to create a DWORD called ShowSecurityPage
3. A value of 1 (Numeric one) means on, whilst 0 (Zero) means off
4. Close the Exchange System Manager, then reopen ESM
So you should now get this:

Without ShowSecurityPage With ShowSecurityPage
I hope this helps.
--wayne