I told you why, i have the powerline adapter av2000 running about.
I cannot make measurements on the same power grid with those adapters running as they communicate using the ground link also so it messes up measurements from the oscilloscope. So when measuring DC instead of say a +12V straight line with the usual noise i get a 2V peak to peak AC wave. I checked all the settings on the oscilloscope as well.
Are you sure what you think is happening is what is actually happening here? (Or stated differently: Are you sure that you don't have something else broken?)
First of all, the scope circuitry itself should be isolated from the AC power line current carrying conductors completely. This is a safety issue if it isn't.
The shield on the scope BNCs will most likely be attached to the ground - BUT... if used properly this shouldn't cause what you're describing - simply because the 'reference' for the measurement should be the ground connector on the probe itself, not the AC power line. So measuring a DC power source should effectively reference the ground potential on the scope, eliminating any noise on the ground line. Think about it this way: The ground noise you're getting in the back becomes the reference for the scope. It doesn't matter what that reference voltage *is*... it could be a very large sine wave.... once you connect ground from the front of the scope to your circuit, the reference voltage for the scope is identical to the reference voltage in your circuit. (I'm ignoring potential safety issues with a large sign wave living on your ground, as that shouldn't ever happen).
Every time I've seen something like this, it's a situation of an improper or missing connection between the circuit under test and the shield on the BNC connectors. Theoretically you could end up with some weird ground currents if your circuit is also ground referenced, and that *might* cause some problems. But almost certainly not at the levels you're describing.
I'd check your scope, and your probes, etc., before going down the path you think you need to go down. Heck, start by finding a chunk of cable with a BNC connector on it and shorting it and then hooking it into the scope input.
I apologize if you've already been down all of these paths, it's just that I'm very skeptical that ground noise on the AC line would cause this problem (with the magnitude you describe) if the cabling on the front of the scope is working and connected properly. I think it's far more likely that either a) your probes are defective (or hooked up wrong) or b) Your scope needs some help. What scope is this? Can you post a pic of say the scope hooked up to a 9V battery and the accompanying waveform?
Specifically in relation to ground isolation: There are ways to do this, but very few are safe. The scope ground is connected to the mains ground for good reason. Only when you understand the reasons behind the connection, and how grounding keeps you safe. Once you understand this, you'll also understand how to remove this isolation if you are willing to accept the risk. I've never felt the need to do so.