I'm currently working on remodeling my office, and putting all the outlets in that office on a new circuit. I have found one existing outlet that is currently fed by a shared-neutral circuit. That is, there is 12-3 leaving the breaker box. The black wire of that 12-3 is connected to one breaker, the red wire is connected to another breaker. The two circuits share the white neutral.

The outlet that I want to remove is fed by the red wire. I have found a junction box where the 12-3 from the breaker box comes out, and the red goes to this outlet while the black carries on. I could simply unhook my outlet in this j-box, and then cap off the live red coming from the breaker box, correct?

However, it looks like this outlet is the only thing that is on this circuit! I can find nothing else. Therefore, I would like to remove this circuit completely, and free up one new spot in the breaker box. What is the proper way to remove the red wire from the breaker box? Note that I can't remove the entire piece of romex, as the black wire and the neutral will still be in service.

Do I just remove the red wire from its breaker? Do I trim it flush somewhere?

Thanks for your help!