If the guppy is already in with the axolotl, there's not much you can do to prevent infection. In future, quarantine fish (as with any new animal) before they go into an established tank. That way, you can spot illness and treat it, if necessary, before the fish goes into a shared tank. If the fish appears unhealthy, it may still be worth removing it to treat it. If the fish appears healthy, then there is probably not much point to removing it at this stage. Is it a fancy guppy? Or a feeder guppy? The conditions that feeder guppies are kept in are usually disease breeding grounds. That said, fancy guppies are a little more specific with temperature requirements than feeders often are, so a fancy might fall ill if your tank gets very cold in the winter.
Guppies shouldn't injure your axolotls, but they will increase the bioload (i.e. waste in the tank) and they will breed like mad. Are you adding fish flakes for them to eat? I would think that, if they get too hungry, they might try nipping on the axolotls' gills, so that's something to watch out for. I have some feeders (that I treated beforehand) that have been breeding in my tank, and I must confess that my axolotls don't bother them often, and certainly eat less than they reproduce. (I will have to start culling/rehoming them soon if they keep breeding at this rate, so that they don't overpopulate the tank.)
And as I'm sure you've read on previous threads, guppies shouldn't be the staple food of your axolotl's diet.
