Help with Group Policy Objects in Server 2008 64-bit

Hi guys,
I need some help with Server 2008 GPOs. This may be a noob question so
be gentle!
I have various servers in OUs and need to prevent users accessing
other servers via a logged on server.
e.g.
User1 logged onto Server A in OU A, must not be able to access Server
B in OU B from Server A, but can logon to Server B directly.
Would dedicated logon accounts be better?
Any ideas?