As a DIYer there are two options, professionals please over rule me as necessary.
1. Remove any flammable debris nearby, put a heat shield (like a baking pan) to protect the flammable wall, make sure your fire extinguisher is handy and use a soldering torch to heat the cleanout evenly. The idea here is to cause the expansion and contraction differences in the metal and rust/oxides to break some of the bonds and make it easier to remove. After it has taken some heat (not red hot, just hot), use a plumbing wrench on the male plug and a wrench on the outside fitting so you can get maximum force (feel free to use breaker bars if you have them and be careful).
If that doesn't work.
2. Using a rust penetrant (my favorite is 50/50 Acetone and ATF), let it soak, and tap around the outside threads with a hammer to help the pentrant get into the threads (tap on both sides at once, and work your way around). Using the same method with the plumbing wrenches get as much force as you can on it, feel free to phone a friend to help.
If you do get it off, replace the metal cap with a plastic one and use several wraps of teflon tape or a liberal amount of teflon paste to seal it so you won't have this problem again.
I take no responsibility for damage to yourself, your ego, property, or others.