Take the knife and scratch off the majority of the Silly Putty. Saturate any leftover putty with WD-40. Allow the clothing to stand for several minutes.
Scrape the clothing to expunge the rest of the Silly Putty. Cover the stained area with WD-40 a second time, then rub the stain with a cotton swab.
Coat another cotton swab in rubbing alcohol. Sponge this onto the stain and then rinse the area of alcohol.
Dampen a cloth with water and liquid dish soap. Mop away whatever is left of the stain.