You're far better off disassembling the gun so the hammer can be worked on unimpeded.
However, you can do a good job by masking off everything but the hammer spur.
You can do this with heavy tape, brass sheet, plastic shim stock, or what have you.
The idea is to cover everything with a protective wrap to prevent damage to the gun or it's finish if you slip.
The masking needs to be saw and file resistant.
If you slip and damage the mask, STOP, and replace the mask.
Hammers are typically pretty hard, especially case hardened hammers which have a glass hard surface.
Most hammers can be cut with a GOOD fine-tooth hacksaw, a cut-off wheel, a diamond saw blade, etc.
After cutting the spur off, radius and smooth the area with a Dremel or sand cloth wrapped around a flat file.
Clean up all the metal filings and sanding dust THOROUGHLY before unmasking the gun, so you don't get abrasive crud in the action.
After everything is clean, test fire AT LEAST 100 rounds to be SURE there's no misfire issues.
Most revolvers work fine with a de-spur job AS LONG as the revolver has had NO "trigger job" work.
If the revolver has had the action worked over, clipping the spur may well bring unreliable operation.