If it's rancid, it's not seasoning; it's just leftover oils. Scrub vigorously with hot water and a little detergent to get the unpolymerized gunk out.
If you want to remove the seasoning, oven cleaner (such as Easy-Off) is good, especially if you seal the pan up wet in a plastic bag after spraying and let it sit for 24 hours. Sandblasting also works well. I know people who've put their cast iron in the oven and run a cleaning cycle, but there's some small risk of warping.
Whatever you do, I always recommend sanding down the bottom of the pan. The smoother the surface, the easier it is to season and the slicker the seasoning layer will be.