It's just a pipe cap. As that guy mentioned, it should just unscrew unless there's a little spot weld on it somewhere. Even then if you wail on it you might just be able to break the weld.
No need to saw it off, just soak the hell out of it with WD40 then get a lock-on monkey wrench (pipe wrench if you like) and smash the wrench with a hammer. If you can't get a good grip on it, file a flat section on each side of the cap.
If you wanna make it easy, go do it in the middle of the night now, before it gets cold and snowy, then just finger-tight it when you put it back on. Remember that when it gets cold the threads will contract slightly and make it tighter than a nun's nasty.
Protip: If you have a little propane torch, give it hell with that for about a minute, just don't mix that with WD40 unless you have the workshop non-flammable stuff.