Amputate before the infection spreads and kills the axolotl; a sharp pair of scissors will do the job. Axolotls grow their limbs back in about 3 months, and amputation probably doesn't hurt them the same way it would hurt us, since their legs have evolved to be disposable, replaceable organs, rather than something vital that has to be protected to avoid being permanently crippled. They're like lizard tails in that respect; they have their uses for crawling around, but a big part of their function is to pop off when something grabs them and then regrow, and they're not something that the animal particularly seems to miss having while it's regrowing.