Well immediate killing of it you can bleach it. It will make it look pretty damn ugly even though it already does lol. Other options that will take care of it are putting lotrimin or any other athletes foot cream. Look on the indications and it should actually say it is used for treating ring worm as well.
Now to prevent... make sure someone is cleaning the mats by bleaching them before each practice and everyone is taking showers. You dont want someone getting it then rolling around with it uncovered and spreading it to the whole team.