You need to create DHCP Class ID's on the Server and assign the DHCP Class ID to each User that should receive the different settings.
I'm not at my DHCP server, so can't tell you exactly how to do it, but an overview is......
Create a DHCP Class ID on the server called GROUPA then assign the default gateway to that CLASSID
Then go to each client that should have Internet Access and set their DHCP CLASSID to GROUPA using netsh command.
This means that when a normal DHCP client requests IP Settings they will not get the default gateway.
If a DHCP client that you have set the DHCP CLASSID to GROUPA requests an IP Address they will also get the Default Gateway.
Don't forget that any user with local administrator rights can simply add the default gateway to their IP settings or set the DHCP CLASSID themselves.