Bobbing the hammer on an SP101 (or GP for that matter) is particularly easy.
The amount of metal weight lost as a percentage of hammer weight is small. The springs on the factory bob-hammer version are the same as a normal version.
Look carefully at the normal hammer versus bobbed, and you'll see that there's a "natural curve" which the thumbrest "juts out of". On a bob-job, all you have to do is "follow the curve around".
*I* recommend a home bob-job as the safer, easier method versus a hammer swap, for two reasons:
1) Finding a factory-lopped hammer may not be easy, as that gun hasn't been shipping long. Check with Brownell's etc. but I'd be willing to bet they don't have 'em.
2) Even if you score one, now you've got a new sear engagement surface to think about. Basically, that whole hammer/trigger mating point is one of the most critical things going on in the whole gun. By lopping the thumbrest off yourself, you avoid having to deal with all that.
My take: lop most of it with a dremel cut-off wheel or hacksaw then switch to progressively finer files, then a knife sharpening stone. Don't forget to completely strip it down and clean it internally of any metal shavings that might have gotten down into the works. See also the SP101 manual on total disassembly (grip frame/action out at a minimum).
Now, there's also the issue of "can it still be cocked SA?" You have three choices there:
a) Me, I'd take the top-most area of the hammer and cut grooves in with a checkering file, so there's still SA capability if you're careful. When lowering the hammer, you need your thumb way up over the top and the pad of your thumb down against the hammer face and finger OFF the trigger ASAP. But then again, in my opinion that's the safest way to decock ALL DA/SA wheelguns. Or straight-up SAs for that matter.
b) Take it apart, work out where the SA cocking notch is on the hammer, and smooth it out into uselessness. You need to have a good mental understanding of what's going on in there to make sure you're grinding out the right thing. Done right, what you've done is made it "uncockable". The alleged advantage (Ayoob et al) is that a prosecutor or lawyer in a civil suit can't claim you "cocked it and gave it a hair trigger" after a shooting.
c) Just leave it, and don't ever cock it.
Final thought: Ruger doesn't sell a 3" barrel lop-hammer variant. I can think of plenty of reasons to build such a thing.