Remove unused wires from circuit breaker panel
Wait, what? Why would 240V 3-wire need the neutral bar??  

Oh, right, this is a main panel where neutral and ground bars are supposed to be bonded, so someone took the lazy but allowed approach of landing all neutrals and grounds in the neutral bar, and not having a ground bar.  So your neutral bar is stuffed.  

The easiest way to free up space on the neutral bar is to get an accessory ground bar for that panel and move a bunch of grounds to it.  

If the hots are all disconnected, why not disconnect the ground? Because some other circuit might be relying on that for ground.  In older homes where everything was not  grounded, it is common for grounds to be retrofit by running a ground from the previously ungrounded receptacle to a grounded site.  While dodgy in the past, this was made officially legal and recommended in the 2014 Code.  

So go to the other end of the wire, and look at the termination of that ground wire and the equipment it is grounding.   If nothing else is using it as a ground, then feel free to detach and cap off both ends of the ground wire.   In the panel, wrap it with tape (preferably green) so it's not at risk for hitting anything hot inside the panel.  

Then take all 3 wires and push them into a corner where they'll be out of the way.  I recommend not tearing them out, as they can be re-tasked for a 120V or 240V-only circuit for any future need.  "Leaving them alone" is way cheaper than fishing new cable.